text: 'use strict';
  
  /*!
   * Module dependencies.
   */
  
  const CastError = require('./error/cast');
  const DocumentNotFoundError = require('./error/notFound');
  const Kareem = require('kareem');
  const MongooseError = require('./error/mongooseError');
  const ObjectParameterError = require('./error/objectParameter');
  const QueryCursor = require('./cursor/QueryCursor');
  const ReadPreference = require('./driver').get().ReadPreference;
  const applyGlobalMaxTimeMS = require('./helpers/query/applyGlobalMaxTimeMS');
  const applyWriteConcern = require('./helpers/schema/applyWriteConcern');
  const cast = require('./cast');
  const castArrayFilters = require('./helpers/update/castArrayFilters');
  const castUpdate = require('./helpers/query/castUpdate');
  const completeMany = require('./helpers/query/completeMany');
  const get = require('./helpers/get');
  const promiseOrCallback = require('./helpers/promiseOrCallback');
  const getDiscriminatorByValue = require('./helpers/discriminator/getDiscriminatorByValue');
  const hasDollarKeys = require('./helpers/query/hasDollarKeys');
  const helpers = require('./queryhelpers');
  const isExclusive = require('./helpers/projection/isExclusive');
  const isInclusive = require('./helpers/projection/isInclusive');
  const mquery = require('mquery');
  const parseProjection = require('./helpers/projection/parseProjection');
  const removeUnusedArrayFilters = require('./helpers/update/removeUnusedArrayFilters');
  const selectPopulatedFields = require('./helpers/query/selectPopulatedFields');
  const setDefaultsOnInsert = require('./helpers/setDefaultsOnInsert');
  const slice = require('sliced');
  const updateValidators = require('./helpers/updateValidators');
  const util = require('util');
  const utils = require('./utils');
  const wrapThunk = require('./helpers/query/wrapThunk');
  
  /**
   * Query constructor used for building queries. You do not need
   * to instantiate a `Query` directly. Instead use Model functions like
   * [`Model.find()`](/docs/api.html#find_find).
   *
   * ####Example:
   *
   *     const query = MyModel.find(); // `query` is an instance of `Query`
   *     query.setOptions({ lean : true });
   *     query.collection(MyModel.collection);
   *     query.where('age').gte(21).exec(callback);
   *
   *     // You can instantiate a query directly. There is no need to do
   *     // this unless you're an advanced user with a very good reason to.
   *     const query = new mongoose.Query();
   *
   * @param {Object} [options]
   * @param {Object} [model]
   * @param {Object} [conditions]
   * @param {Object} [collection] Mongoose collection
   * @api public
   */
  
  function Query(conditions, options, model, collection) {
    // this stuff is for dealing with custom queries created by #toConstructor
    if (!this._mongooseOptions) {
      this._mongooseOptions = {};
    }
    options = options || {};
  
    this._transforms = [];
    this._hooks = new Kareem();
    this._executionCount = 0;
  
    // this is the case where we have a CustomQuery, we need to check if we got
    // options passed in, and if we did, merge them in
    const keys = Object.keys(options);
    for (const key of keys) {
      this._mongooseOptions[key] = options[key];
    }
  
    if (collection) {
      this.mongooseCollection = collection;
    }
  
    if (model) {
      this.model = model;
      this.schema = model.schema;
    }
  
  
    // this is needed because map reduce returns a model that can be queried, but
    // all of the queries on said model should be lean
    if (this.model && this.model._mapreduce) {
      this.lean();
    }
  
    // inherit mquery
    mquery.call(this, this.mongooseCollection, options);
  
    if (conditions) {
      this.find(conditions);
    }
  
    this.options = this.options || {};
  
    // For gh-6880. mquery still needs to support `fields` by default for old
    // versions of MongoDB
    this.$useProjection = true;
  
    const collation = get(this, 'schema.options.collation', null);
    if (collation != null) {
      this.options.collation = collation;
    }
  }
  
  /*!
   * inherit mquery
   */
  
  Query.prototype = new mquery;
  Query.prototype.constructor = Query;
  Query.base = mquery.prototype;
  
  /**
   * Flag to opt out of using `$geoWithin`.
   *
   *     mongoose.Query.use$geoWithin = false;
   *
   * MongoDB 2.4 deprecated the use of `$within`, replacing it with `$geoWithin`. Mongoose uses `$geoWithin` by default (which is 100% backward compatible with `$within`). If you are running an older version of MongoDB, set this flag to `false` so your `within()` queries continue to work.
   *
   * @see http://docs.mongodb.org/manual/reference/operator/geoWithin/
   * @default true
   * @property use$geoWithin
   * @memberOf Query
   * @receiver Query
   * @api public
   */
  
  Query.use$geoWithin = mquery.use$geoWithin;
  
  /**
   * Converts this query to a customized, reusable query constructor with all arguments and options retained.
   *
   * ####Example
   *
   *     // Create a query for adventure movies and read from the primary
   *     // node in the replica-set unless it is down, in which case we'll
   *     // read from a secondary node.
   *     const query = Movie.find({ tags: 'adventure' }).read('primaryPreferred');
   *
   *     // create a custom Query constructor based off these settings
   *     const Adventure = query.toConstructor();
   *
   *     // Adventure is now a subclass of mongoose.Query and works the same way but with the
   *     // default query parameters and options set.
   *     Adventure().exec(callback)
   *
   *     // further narrow down our query results while still using the previous settings
   *     Adventure().where({ name: /^Life/ }).exec(callback);
   *
   *     // since Adventure is a stand-alone constructor we can also add our own
   *     // helper methods and getters without impacting global queries
   *     Adventure.prototype.startsWith = function (prefix) {
   *       this.where({ name: new RegExp('^' + prefix) })
   *       return this;
   *     }
   *     Object.defineProperty(Adventure.prototype, 'highlyRated', {
   *       get: function () {
   *         this.where({ rating: { $gt: 4.5 }});
   *         return this;
   *       }
   *     })
   *     Adventure().highlyRated.startsWith('Life').exec(callback)
   *
   * @return {Query} subclass-of-Query
   * @api public
   */
  
  Query.prototype.toConstructor = function toConstructor() {
    const model = this.model;
    const coll = this.mongooseCollection;
  
    const CustomQuery = function(criteria, options) {
      if (!(this instanceof CustomQuery)) {
        return new CustomQuery(criteria, options);
      }
      this._mongooseOptions = utils.clone(p._mongooseOptions);
      Query.call(this, criteria, options || null, model, coll);
    };
  
    util.inherits(CustomQuery, model.Query);
  
    // set inherited defaults
    const p = CustomQuery.prototype;
  
    p.options = {};
  
    // Need to handle `sort()` separately because entries-style `sort()` syntax
    // `sort([['prop1', 1]])` confuses mquery into losing the outer nested array.
    // See gh-8159
    const options = Object.assign({}, this.options);
    if (options.sort != null) {
      p.sort(options.sort);
      delete options.sort;
    }
    p.setOptions(options);
  
    p.op = this.op;
    p._conditions = utils.clone(this._conditions);
    p._fields = utils.clone(this._fields);
    p._update = utils.clone(this._update, {
      flattenDecimals: false
    });
    p._path = this._path;
    p._distinct = this._distinct;
    p._collection = this._collection;
    p._mongooseOptions = this._mongooseOptions;
  
    return CustomQuery;
  };
  
  /**
   * Specifies a javascript function or expression to pass to MongoDBs query system.
   *
   * ####Example
   *
   *     query.$where('this.comments.length === 10 || this.name.length === 5')
   *
   *     // or
   *
   *     query.$where(function () {
   *       return this.comments.length === 10 || this.name.length === 5;
   *     })
   *
   * ####NOTE:
   *
   * Only use `$where` when you have a condition that cannot be met using other MongoDB operators like `$lt`.
   * **Be sure to read about all of [its caveats](http://docs.mongodb.org/manual/reference/operator/where/) before using.**
   *
   * @see $where http://docs.mongodb.org/manual/reference/operator/where/
   * @method $where
   * @param {String|Function} js javascript string or function
   * @return {Query} this
   * @memberOf Query
   * @instance
   * @method $where
   * @api public
   */
  
  /**
   * Specifies a `path` for use with chaining.
   *
   * ####Example
   *
   *     // instead of writing:
   *     User.find({age: {$gte: 21, $lte: 65}}, callback);
   *
   *     // we can instead write:
   *     User.where('age').gte(21).lte(65);
   *
   *     // passing query conditions is permitted
   *     User.find().where({ name: 'vonderful' })
   *
   *     // chaining
   *     User
   *     .where('age').gte(21).lte(65)
   *     .where('name', /^vonderful/i)
   *     .where('friends').slice(10)
   *     .exec(callback)
   *
   * @method where
   * @memberOf Query
   * @instance
   * @param {String|Object} [path]
   * @param {any} [val]
   * @return {Query} this
   * @api public
   */
  
  /**
   * Specifies a `$slice` projection for an array.
   *
   * ####Example
   *
   *     query.slice('comments', 5)
   *     query.slice('comments', -5)
   *     query.slice('comments', [10, 5])
   *     query.where('comments').slice(5)
   *     query.where('comments').slice([-10, 5])
   *
   * @method slice
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Number} val number/range of elements to slice
   * @return {Query} this
   * @see mongodb http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields#RetrievingaSubsetofFields-RetrievingaSubrangeofArrayElements
   * @see $slice http://docs.mongodb.org/manual/reference/projection/slice/#prj._S_slice
   * @api public
   */
  
  Query.prototype.slice = function() {
    if (arguments.length === 0) {
      return this;
    }
  
    this._validate('slice');
  
    let path;
    let val;
  
    if (arguments.length === 1) {
      const arg = arguments[0];
      if (typeof arg === 'object' && !Array.isArray(arg)) {
        const keys = Object.keys(arg);
        const numKeys = keys.length;
        for (let i = 0; i < numKeys; ++i) {
          this.slice(keys[i], arg[keys[i]]);
        }
        return this;
      }
      this._ensurePath('slice');
      path = this._path;
      val = arguments[0];
    } else if (arguments.length === 2) {
      if ('number' === typeof arguments[0]) {
        this._ensurePath('slice');
        path = this._path;
        val = slice(arguments);
      } else {
        path = arguments[0];
        val = arguments[1];
      }
    } else if (arguments.length === 3) {
      path = arguments[0];
      val = slice(arguments, 1);
    }
  
    const p = {};
    p[path] = { $slice: val };
    this.select(p);
  
    return this;
  };
  
  
  /**
   * Specifies the complementary comparison value for paths specified with `where()`
   *
   * ####Example
   *
   *     User.where('age').equals(49);
   *
   *     // is the same as
   *
   *     User.where('age', 49);
   *
   * @method equals
   * @memberOf Query
   * @instance
   * @param {Object} val
   * @return {Query} this
   * @api public
   */
  
  /**
   * Specifies arguments for an `$or` condition.
   *
   * ####Example
   *
   *     query.or([{ color: 'red' }, { status: 'emergency' }])
   *
   * @see $or http://docs.mongodb.org/manual/reference/operator/or/
   * @method or
   * @memberOf Query
   * @instance
   * @param {Array} array array of conditions
   * @return {Query} this
   * @api public
   */
  
  /**
   * Specifies arguments for a `$nor` condition.
   *
   * ####Example
   *
   *     query.nor([{ color: 'green' }, { status: 'ok' }])
   *
   * @see $nor http://docs.mongodb.org/manual/reference/operator/nor/
   * @method nor
   * @memberOf Query
   * @instance
   * @param {Array} array array of conditions
   * @return {Query} this
   * @api public
   */
  
  /**
   * Specifies arguments for a `$and` condition.
   *
   * ####Example
   *
   *     query.and([{ color: 'green' }, { status: 'ok' }])
   *
   * @method and
   * @memberOf Query
   * @instance
   * @see $and http://docs.mongodb.org/manual/reference/operator/and/
   * @param {Array} array array of conditions
   * @return {Query} this
   * @api public
   */
  
  /**
   * Specifies a `$gt` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * ####Example
   *
   *     Thing.find().where('age').gt(21)
   *
   *     // or
   *     Thing.find().gt('age', 21)
   *
   * @method gt
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Number} val
   * @see $gt http://docs.mongodb.org/manual/reference/operator/gt/
   * @api public
   */
  
  /**
   * Specifies a `$gte` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * @method gte
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Number} val
   * @see $gte http://docs.mongodb.org/manual/reference/operator/gte/
   * @api public
   */
  
  /**
   * Specifies a `$lt` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * @method lt
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Number} val
   * @see $lt http://docs.mongodb.org/manual/reference/operator/lt/
   * @api public
   */
  
  /**
   * Specifies a `$lte` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * @method lte
   * @see $lte http://docs.mongodb.org/manual/reference/operator/lte/
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Number} val
   * @api public
   */
  
  /**
   * Specifies a `$ne` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * @see $ne http://docs.mongodb.org/manual/reference/operator/ne/
   * @method ne
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {any} val
   * @api public
   */
  
  /**
   * Specifies an `$in` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * @see $in http://docs.mongodb.org/manual/reference/operator/in/
   * @method in
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Array} val
   * @api public
   */
  
  /**
   * Specifies an `$nin` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * @see $nin http://docs.mongodb.org/manual/reference/operator/nin/
   * @method nin
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Array} val
   * @api public
   */
  
  /**
   * Specifies an `$all` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * ####Example:
   *
   *     MyModel.find().where('pets').all(['dog', 'cat', 'ferret']);
   *     // Equivalent:
   *     MyModel.find().all('pets', ['dog', 'cat', 'ferret']);
   *
   * @see $all http://docs.mongodb.org/manual/reference/operator/all/
   * @method all
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Array} val
   * @api public
   */
  
  /**
   * Specifies a `$size` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * ####Example
   *
   *     MyModel.where('tags').size(0).exec(function (err, docs) {
   *       if (err) return handleError(err);
   *
   *       assert(Array.isArray(docs));
   *       console.log('documents with 0 tags', docs);
   *     })
   *
   * @see $size http://docs.mongodb.org/manual/reference/operator/size/
   * @method size
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Number} val
   * @api public
   */
  
  /**
   * Specifies a `$regex` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * @see $regex http://docs.mongodb.org/manual/reference/operator/regex/
   * @method regex
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {String|RegExp} val
   * @api public
   */
  
  /**
   * Specifies a `maxDistance` query condition.
   *
   * When called with one argument, the most recent path passed to `where()` is used.
   *
   * @see $maxDistance http://docs.mongodb.org/manual/reference/operator/maxDistance/
   * @method maxDistance
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Number} val
   * @api public
   */
  
  /**
   * Specifies a `$mod` condition, filters documents for documents whose
   * `path` property is a number that is equal to `remainder` modulo `divisor`.
   *
   * ####Example
   *
   *     // All find products whose inventory is odd
   *     Product.find().mod('inventory', [2, 1]);
   *     Product.find().where('inventory').mod([2, 1]);
   *     // This syntax is a little strange, but supported.
   *     Product.find().where('inventory').mod(2, 1);
   *
   * @method mod
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Array} val must be of length 2, first element is `divisor`, 2nd element is `remainder`.
   * @return {Query} this
   * @see $mod http://docs.mongodb.org/manual/reference/operator/mod/
   * @api public
   */
  
  Query.prototype.mod = function() {
    let val;
    let path;
  
    if (arguments.length === 1) {
      this._ensurePath('mod');
      val = arguments[0];
      path = this._path;
    } else if (arguments.length === 2 && !Array.isArray(arguments[1])) {
      this._ensurePath('mod');
      val = slice(arguments);
      path = this._path;
    } else if (arguments.length === 3) {
      val = slice(arguments, 1);
      path = arguments[0];
    } else {
      val = arguments[1];
      path = arguments[0];
    }
  
    const conds = this._conditions[path] || (this._conditions[path] = {});
    conds.$mod = val;
    return this;
  };
  
  /**
   * Specifies an `$exists` condition
   *
   * ####Example
   *
   *     // { name: { $exists: true }}
   *     Thing.where('name').exists()
   *     Thing.where('name').exists(true)
   *     Thing.find().exists('name')
   *
   *     // { name: { $exists: false }}
   *     Thing.where('name').exists(false);
   *     Thing.find().exists('name', false);
   *
   * @method exists
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Boolean} val
   * @return {Query} this
   * @see $exists http://docs.mongodb.org/manual/reference/operator/exists/
   * @api public
   */
  
  /**
   * Specifies an `$elemMatch` condition
   *
   * ####Example
   *
   *     query.elemMatch('comment', { author: 'autobot', votes: {$gte: 5}})
   *
   *     query.where('comment').elemMatch({ author: 'autobot', votes: {$gte: 5}})
   *
   *     query.elemMatch('comment', function (elem) {
   *       elem.where('author').equals('autobot');
   *       elem.where('votes').gte(5);
   *     })
   *
   *     query.where('comment').elemMatch(function (elem) {
   *       elem.where({ author: 'autobot' });
   *       elem.where('votes').gte(5);
   *     })
   *
   * @method elemMatch
   * @memberOf Query
   * @instance
   * @param {String|Object|Function} path
   * @param {Object|Function} filter
   * @return {Query} this
   * @see $elemMatch http://docs.mongodb.org/manual/reference/operator/elemMatch/
   * @api public
   */
  
  /**
   * Defines a `$within` or `$geoWithin` argument for geo-spatial queries.
   *
   * ####Example
   *
   *     query.where(path).within().box()
   *     query.where(path).within().circle()
   *     query.where(path).within().geometry()
   *
   *     query.where('loc').within({ center: [50,50], radius: 10, unique: true, spherical: true });
   *     query.where('loc').within({ box: [[40.73, -73.9], [40.7, -73.988]] });
   *     query.where('loc').within({ polygon: [[],[],[],[]] });
   *
   *     query.where('loc').within([], [], []) // polygon
   *     query.where('loc').within([], []) // box
   *     query.where('loc').within({ type: 'LineString', coordinates: [...] }); // geometry
   *
   * **MUST** be used after `where()`.
   *
   * ####NOTE:
   *
   * As of Mongoose 3.7, `$geoWithin` is always used for queries. To change this behavior, see [Query.use$geoWithin](#query_Query-use%2524geoWithin).
   *
   * ####NOTE:
   *
   * In Mongoose 3.7, `within` changed from a getter to a function. If you need the old syntax, use [this](https://github.com/ebensing/mongoose-within).
   *
   * @method within
   * @see $polygon http://docs.mongodb.org/manual/reference/operator/polygon/
   * @see $box http://docs.mongodb.org/manual/reference/operator/box/
   * @see $geometry http://docs.mongodb.org/manual/reference/operator/geometry/
   * @see $center http://docs.mongodb.org/manual/reference/operator/center/
   * @see $centerSphere http://docs.mongodb.org/manual/reference/operator/centerSphere/
   * @memberOf Query
   * @instance
   * @return {Query} this
   * @api public
   */
  
  /**
   * Specifies the maximum number of documents the query will return.
   *
   * ####Example
   *
   *     query.limit(20)
   *
   * ####Note
   *
   * Cannot be used with `distinct()`
   *
   * @method limit
   * @memberOf Query
   * @instance
   * @param {Number} val
   * @api public
   */
  
  /**
   * Specifies the number of documents to skip.
   *
   * ####Example
   *
   *     query.skip(100).limit(20)
   *
   * ####Note
   *
   * Cannot be used with `distinct()`
   *
   * @method skip
   * @memberOf Query
   * @instance
   * @param {Number} val
   * @see cursor.skip http://docs.mongodb.org/manual/reference/method/cursor.skip/
   * @api public
   */
  
  /**
   * Specifies the maxScan option.
   *
   * ####Example
   *
   *     query.maxScan(100)
   *
   * ####Note
   *
   * Cannot be used with `distinct()`
   *
   * @method maxScan
   * @memberOf Query
   * @instance
   * @param {Number} val
   * @see maxScan http://docs.mongodb.org/manual/reference/operator/maxScan/
   * @api public
   */
  
  /**
   * Specifies the batchSize option.
   *
   * ####Example
   *
   *     query.batchSize(100)
   *
   * ####Note
   *
   * Cannot be used with `distinct()`
   *
   * @method batchSize
   * @memberOf Query
   * @instance
   * @param {Number} val
   * @see batchSize http://docs.mongodb.org/manual/reference/method/cursor.batchSize/
   * @api public
   */
  
  /**
   * Specifies the `comment` option.
   *
   * ####Example
   *
   *     query.comment('login query')
   *
   * ####Note
   *
   * Cannot be used with `distinct()`
   *
   * @method comment
   * @memberOf Query
   * @instance
   * @param {String} val
   * @see comment http://docs.mongodb.org/manual/reference/operator/comment/
   * @api public
   */
  
  /**
   * Specifies this query as a `snapshot` query.
   *
   * ####Example
   *
   *     query.snapshot() // true
   *     query.snapshot(true)
   *     query.snapshot(false)
   *
   * ####Note
   *
   * Cannot be used with `distinct()`
   *
   * @method snapshot
   * @memberOf Query
   * @instance
   * @see snapshot http://docs.mongodb.org/manual/reference/operator/snapshot/
   * @return {Query} this
   * @api public
   */
  
  /**
   * Sets query hints.
   *
   * ####Example
   *
   *     query.hint({ indexA: 1, indexB: -1})
   *
   * ####Note
   *
   * Cannot be used with `distinct()`
   *
   * @method hint
   * @memberOf Query
   * @instance
   * @param {Object} val a hint object
   * @return {Query} this
   * @see $hint http://docs.mongodb.org/manual/reference/operator/hint/
   * @api public
   */
  
  /**
   * Get/set the current projection (AKA fields). Pass `null` to remove the
   * current projection.
   *
   * Unlike `projection()`, the `select()` function modifies the current
   * projection in place. This function overwrites the existing projection.
   *
   * ####Example:
   *
   *     const q = Model.find();
   *     q.projection(); // null
   *
   *     q.select('a b');
   *     q.projection(); // { a: 1, b: 1 }
   *
   *     q.projection({ c: 1 });
   *     q.projection(); // { c: 1 }
   *
   *     q.projection(null);
   *     q.projection(); // null
   *
   *
   * @method projection
   * @memberOf Query
   * @instance
   * @param {Object|null} arg
   * @return {Object} the current projection
   * @api public
   */
  
  Query.prototype.projection = function(arg) {
    if (arguments.length === 0) {
      return this._fields;
    }
  
    this._fields = {};
    this._userProvidedFields = {};
    this.select(arg);
    return this._fields;
  };
  
  /**
   * Specifies which document fields to include or exclude (also known as the query "projection")
   *
   * When using string syntax, prefixing a path with `-` will flag that path as excluded. When a path does not have the `-` prefix, it is included. Lastly, if a path is prefixed with `+`, it forces inclusion of the path, which is useful for paths excluded at the [schema level](/docs/api.html#schematype_SchemaType-select).
   *
   * A projection _must_ be either inclusive or exclusive. In other words, you must
   * either list the fields to include (which excludes all others), or list the fields
   * to exclude (which implies all other fields are included). The [`_id` field is the only exception because MongoDB includes it by default](https://docs.mongodb.com/manual/tutorial/project-fields-from-query-results/#suppress-id-field).
   *
   * ####Example
   *
   *     // include a and b, exclude other fields
   *     query.select('a b');
   *     // Equivalent syntaxes:
   *     query.select(['a', 'b']);
   *     query.select({ a: 1, b: 1 });
   *
   *     // exclude c and d, include other fields
   *     query.select('-c -d');
   *
   *     // Use `+` to override schema-level `select: false` without making the
   *     // projection inclusive.
   *     const schema = new Schema({
   *       foo: { type: String, select: false },
   *       bar: String
   *     });
   *     // ...
   *     query.select('+foo'); // Override foo's `select: false` without excluding `bar`
   *
   *     // or you may use object notation, useful when
   *     // you have keys already prefixed with a "-"
   *     query.select({ a: 1, b: 1 });
   *     query.select({ c: 0, d: 0 });
   *
   *
   * @method select
   * @memberOf Query
   * @instance
   * @param {Object|String|Array<String>} arg
   * @return {Query} this
   * @see SchemaType
   * @api public
   */
  
  Query.prototype.select = function select() {
    let arg = arguments[0];
    if (!arg) return this;
  
    if (arguments.length !== 1) {
      throw new Error('Invalid select: select only takes 1 argument');
    }
  
    this._validate('select');
  
    const fields = this._fields || (this._fields = {});
    const userProvidedFields = this._userProvidedFields || (this._userProvidedFields = {});
  
    arg = parseProjection(arg);
  
    if (utils.isObject(arg)) {
      const keys = Object.keys(arg);
      for (let i = 0; i < keys.length; ++i) {
        let value = arg[keys[i]];
        // Workaround for gh-10142
        if (typeof value === 'string') {
          value = 1;
        }
        fields[keys[i]] = value;
        userProvidedFields[keys[i]] = value;
      }
      return this;
    }
  
    throw new TypeError('Invalid select() argument. Must be string or object.');
  };
  
  /**
   * _DEPRECATED_ Sets the slaveOk option.
   *
   * **Deprecated** in MongoDB 2.2 in favor of [read preferences](#query_Query-read).
   *
   * ####Example:
   *
   *     query.slaveOk() // true
   *     query.slaveOk(true)
   *     query.slaveOk(false)
   *
   * @method slaveOk
   * @memberOf Query
   * @instance
   * @deprecated use read() preferences instead if on mongodb >= 2.2
   * @param {Boolean} v defaults to true
   * @see mongodb http://docs.mongodb.org/manual/applications/replication/#read-preference
   * @see slaveOk http://docs.mongodb.org/manual/reference/method/rs.slaveOk/
   * @see read() #query_Query-read
   * @return {Query} this
   * @api public
   */
  
  /**
   * Determines the MongoDB nodes from which to read.
   *
   * ####Preferences:
   *
   *     primary - (default) Read from primary only. Operations will produce an error if primary is unavailable. Cannot be combined with tags.
   *     secondary            Read from secondary if available, otherwise error.
   *     primaryPreferred     Read from primary if available, otherwise a secondary.
   *     secondaryPreferred   Read from a secondary if available, otherwise read from the primary.
   *     nearest              All operations read from among the nearest candidates, but unlike other modes, this option will include both the primary and all secondaries in the random selection.
   *
   * Aliases
   *
   *     p   primary
   *     pp  primaryPreferred
   *     s   secondary
   *     sp  secondaryPreferred
   *     n   nearest
   *
   * ####Example:
   *
   *     new Query().read('primary')
   *     new Query().read('p')  // same as primary
   *
   *     new Query().read('primaryPreferred')
   *     new Query().read('pp') // same as primaryPreferred
   *
   *     new Query().read('secondary')
   *     new Query().read('s')  // same as secondary
   *
   *     new Query().read('secondaryPreferred')
   *     new Query().read('sp') // same as secondaryPreferred
   *
   *     new Query().read('nearest')
   *     new Query().read('n')  // same as nearest
   *
   *     // read from secondaries with matching tags
   *     new Query().read('s', [{ dc:'sf', s: 1 },{ dc:'ma', s: 2 }])
   *
   * Read more about how to use read preferrences [here](http://docs.mongodb.org/manual/applications/replication/#read-preference) and [here](http://mongodb.github.com/node-mongodb-native/driver-articles/anintroductionto1_1and2_2.html#read-preferences).
   *
   * @method read
   * @memberOf Query
   * @instance
   * @param {String} pref one of the listed preference options or aliases
   * @param {Array} [tags] optional tags for this query
   * @see mongodb http://docs.mongodb.org/manual/applications/replication/#read-preference
   * @see driver http://mongodb.github.com/node-mongodb-native/driver-articles/anintroductionto1_1and2_2.html#read-preferences
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.read = function read(pref, tags) {
    // first cast into a ReadPreference object to support tags
    const read = new ReadPreference(pref, tags);
    this.options.readPreference = read;
    return this;
  };
  
  /**
   * Sets the [MongoDB session](https://docs.mongodb.com/manual/reference/server-sessions/)
   * associated with this query. Sessions are how you mark a query as part of a
   * [transaction](/docs/transactions.html).
   *
   * Calling `session(null)` removes the session from this query.
   *
   * ####Example:
   *
   *     const s = await mongoose.startSession();
   *     await mongoose.model('Person').findOne({ name: 'Axl Rose' }).session(s);
   *
   * @method session
   * @memberOf Query
   * @instance
   * @param {ClientSession} [session] from `await conn.startSession()`
   * @see Connection.prototype.startSession() /docs/api.html#connection_Connection-startSession
   * @see mongoose.startSession() /docs/api.html#mongoose_Mongoose-startSession
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.session = function session(v) {
    if (v == null) {
      delete this.options.session;
    }
    this.options.session = v;
    return this;
  };
  
  /**
   * Sets the 3 write concern parameters for this query:
   *
   * - `w`: Sets the specified number of `mongod` servers, or tag set of `mongod` servers, that must acknowledge this write before this write is considered successful.
   * - `j`: Boolean, set to `true` to request acknowledgement that this operation has been persisted to MongoDB's on-disk journal.
   * - `wtimeout`: If [`w > 1`](/docs/api.html#query_Query-w), the maximum amount of time to wait for this write to propagate through the replica set before this operation fails. The default is `0`, which means no timeout.
   *
   * This option is only valid for operations that write to the database:
   *
   * - `deleteOne()`
   * - `deleteMany()`
   * - `findOneAndDelete()`
   * - `findOneAndReplace()`
   * - `findOneAndUpdate()`
   * - `remove()`
   * - `update()`
   * - `updateOne()`
   * - `updateMany()`
   *
   * Defaults to the schema's [`writeConcern` option](/docs/guide.html#writeConcern)
   *
   * ####Example:
   *
   *     // The 'majority' option means the `deleteOne()` promise won't resolve
   *     // until the `deleteOne()` has propagated to the majority of the replica set
   *     await mongoose.model('Person').
   *       deleteOne({ name: 'Ned Stark' }).
   *       writeConcern({ w: 'majority' });
   *
   * @method writeConcern
   * @memberOf Query
   * @instance
   * @param {Object} writeConcern the write concern value to set
   * @see mongodb https://mongodb.github.io/node-mongodb-native/3.1/api/global.html#WriteConcern
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.writeConcern = function writeConcern(val) {
    if (val == null) {
      delete this.options.writeConcern;
      return this;
    }
    this.options.writeConcern = val;
    return this;
  };
  
  /**
   * Sets the specified number of `mongod` servers, or tag set of `mongod` servers,
   * that must acknowledge this write before this write is considered successful.
   * This option is only valid for operations that write to the database:
   *
   * - `deleteOne()`
   * - `deleteMany()`
   * - `findOneAndDelete()`
   * - `findOneAndReplace()`
   * - `findOneAndUpdate()`
   * - `remove()`
   * - `update()`
   * - `updateOne()`
   * - `updateMany()`
   *
   * Defaults to the schema's [`writeConcern.w` option](/docs/guide.html#writeConcern)
   *
   * ####Example:
   *
   *     // The 'majority' option means the `deleteOne()` promise won't resolve
   *     // until the `deleteOne()` has propagated to the majority of the replica set
   *     await mongoose.model('Person').
   *       deleteOne({ name: 'Ned Stark' }).
   *       w('majority');
   *
   * @method w
   * @memberOf Query
   * @instance
   * @param {String|number} val 0 for fire-and-forget, 1 for acknowledged by one server, 'majority' for majority of the replica set, or [any of the more advanced options](https://docs.mongodb.com/manual/reference/write-concern/#w-option).
   * @see mongodb https://docs.mongodb.com/manual/reference/write-concern/#w-option
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.w = function w(val) {
    if (val == null) {
      delete this.options.w;
    }
    if (this.options.writeConcern != null) {
      this.options.writeConcern.w = val;
    } else {
      this.options.w = val;
    }
    return this;
  };
  
  /**
   * Requests acknowledgement that this operation has been persisted to MongoDB's
   * on-disk journal.
   * This option is only valid for operations that write to the database:
   *
   * - `deleteOne()`
   * - `deleteMany()`
   * - `findOneAndDelete()`
   * - `findOneAndReplace()`
   * - `findOneAndUpdate()`
   * - `remove()`
   * - `update()`
   * - `updateOne()`
   * - `updateMany()`
   *
   * Defaults to the schema's [`writeConcern.j` option](/docs/guide.html#writeConcern)
   *
   * ####Example:
   *
   *     await mongoose.model('Person').deleteOne({ name: 'Ned Stark' }).j(true);
   *
   * @method j
   * @memberOf Query
   * @instance
   * @param {boolean} val
   * @see mongodb https://docs.mongodb.com/manual/reference/write-concern/#j-option
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.j = function j(val) {
    if (val == null) {
      delete this.options.j;
    }
    if (this.options.writeConcern != null) {
      this.options.writeConcern.j = val;
    } else {
      this.options.j = val;
    }
    return this;
  };
  
  /**
   * If [`w > 1`](/docs/api.html#query_Query-w), the maximum amount of time to
   * wait for this write to propagate through the replica set before this
   * operation fails. The default is `0`, which means no timeout.
   *
   * This option is only valid for operations that write to the database:
   *
   * - `deleteOne()`
   * - `deleteMany()`
   * - `findOneAndDelete()`
   * - `findOneAndReplace()`
   * - `findOneAndUpdate()`
   * - `remove()`
   * - `update()`
   * - `updateOne()`
   * - `updateMany()`
   *
   * Defaults to the schema's [`writeConcern.wtimeout` option](/docs/guide.html#writeConcern)
   *
   * ####Example:
   *
   *     // The `deleteOne()` promise won't resolve until this `deleteOne()` has
   *     // propagated to at least `w = 2` members of the replica set. If it takes
   *     // longer than 1 second, this `deleteOne()` will fail.
   *     await mongoose.model('Person').
   *       deleteOne({ name: 'Ned Stark' }).
   *       w(2).
   *       wtimeout(1000);
   *
   * @method wtimeout
   * @memberOf Query
   * @instance
   * @param {number} ms number of milliseconds to wait
   * @see mongodb https://docs.mongodb.com/manual/reference/write-concern/#wtimeout
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.wtimeout = function wtimeout(ms) {
    if (ms == null) {
      delete this.options.wtimeout;
    }
    if (this.options.writeConcern != null) {
      this.options.writeConcern.wtimeout = ms;
    } else {
      this.options.wtimeout = ms;
    }
    return this;
  };
  
  /**
   * Sets the readConcern option for the query.
   *
   * ####Example:
   *
   *     new Query().readConcern('local')
   *     new Query().readConcern('l')  // same as local
   *
   *     new Query().readConcern('available')
   *     new Query().readConcern('a')  // same as available
   *
   *     new Query().readConcern('majority')
   *     new Query().readConcern('m')  // same as majority
   *
   *     new Query().readConcern('linearizable')
   *     new Query().readConcern('lz') // same as linearizable
   *
   *     new Query().readConcern('snapshot')
   *     new Query().readConcern('s')  // same as snapshot
   *
   *
   * ####Read Concern Level:
   *
   *     local         MongoDB 3.2+ The query returns from the instance with no guarantee guarantee that the data has been written to a majority of the replica set members (i.e. may be rolled back).
   *     available     MongoDB 3.6+ The query returns from the instance with no guarantee guarantee that the data has been written to a majority of the replica set members (i.e. may be rolled back).
   *     majority      MongoDB 3.2+ The query returns the data that has been acknowledged by a majority of the replica set members. The documents returned by the read operation are durable, even in the event of failure.
   *     linearizable  MongoDB 3.4+ The query returns data that reflects all successful majority-acknowledged writes that completed prior to the start of the read operation. The query may wait for concurrently executing writes to propagate to a majority of replica set members before returning results.
   *     snapshot      MongoDB 4.0+ Only available for operations within multi-document transactions. Upon transaction commit with write concern "majority", the transaction operations are guaranteed to have read from a snapshot of majority-committed data.
   *
   * Aliases
   *
   *     l   local
   *     a   available
   *     m   majority
   *     lz  linearizable
   *     s   snapshot
   *
   * Read more about how to use read concern [here](https://docs.mongodb.com/manual/reference/read-concern/).
   *
   * @memberOf Query
   * @method readConcern
   * @param {String} level one of the listed read concern level or their aliases
   * @see mongodb https://docs.mongodb.com/manual/reference/read-concern/
   * @return {Query} this
   * @api public
   */
  
  /**
   * Gets query options.
   *
   * ####Example:
   *
   *     const query = new Query();
   *     query.limit(10);
   *     query.setOptions({ maxTimeMS: 1000 })
   *     query.getOptions(); // { limit: 10, maxTimeMS: 1000 }
   *
   * @return {Object} the options
   * @api public
   */
  
  Query.prototype.getOptions = function() {
    return this.options;
  };
  
  /**
   * Sets query options. Some options only make sense for certain operations.
   *
   * ####Options:
   *
   * The following options are only for `find()`:
   *
   * - [tailable](http://www.mongodb.org/display/DOCS/Tailable+Cursors)
   * - [sort](http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%7B%7Bsort(\)%7D%7D)
   * - [limit](http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%7B%7Blimit%28%29%7D%7D)
   * - [skip](http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%7B%7Bskip%28%29%7D%7D)
   * - [allowDiskUse](https://docs.mongodb.com/manual/reference/method/cursor.allowDiskUse/)
   * - [batchSize](http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%7B%7BbatchSize%28%29%7D%7D)
   * - [readPreference](http://docs.mongodb.org/manual/applications/replication/#read-preference)
   * - [hint](http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%24hint)
   * - [comment](http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%24comment)
   * - [snapshot](http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%7B%7Bsnapshot%28%29%7D%7D)
   * - [maxscan](https://docs.mongodb.org/v3.2/reference/operator/meta/maxScan/#metaOp._S_maxScan)
   *
   * The following options are only for write operations: `update()`, `updateOne()`, `updateMany()`, `replaceOne()`, `findOneAndUpdate()`, and `findByIdAndUpdate()`:
   *
   * - [upsert](https://docs.mongodb.com/manual/reference/method/db.collection.update/)
   * - [writeConcern](https://docs.mongodb.com/manual/reference/method/db.collection.update/)
   * - [timestamps](https://mongoosejs.com/docs/guide.html#timestamps): If `timestamps` is set in the schema, set this option to `false` to skip timestamps for that particular update. Has no effect if `timestamps` is not enabled in the schema options.
   * - omitUndefined: delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
   * - overwriteDiscriminatorKey: allow setting the discriminator key in the update. Will use the correct discriminator schema if the update changes the discriminator key.
   * - overwrite: replace the entire document
   *
   * The following options are only for `find()`, `findOne()`, `findById()`, `findOneAndUpdate()`, and `findByIdAndUpdate()`:
   *
   * - [lean](./api.html#query_Query-lean)
   * - [populate](/docs/populate.html)
   * - [projection](/docs/api/query.html#query_Query-projection)
   *
   * The following options are only for all operations **except** `update()`, `updateOne()`, `updateMany()`, `remove()`, `deleteOne()`, and `deleteMany()`:
   *
   * - [maxTimeMS](https://docs.mongodb.com/manual/reference/operator/meta/maxTimeMS/)
   *
   * The following options are for `findOneAndUpdate()` and `findOneAndRemove()`
   *
   * - [useFindAndModify](/docs/deprecations.html#findandmodify)
   * - rawResult
   *
   * The following options are for all operations:
   *
   * - [collation](https://docs.mongodb.com/manual/reference/collation/)
   * - [session](https://docs.mongodb.com/manual/reference/server-sessions/)
   * - [explain](https://docs.mongodb.com/manual/reference/method/cursor.explain/)
   *
   * @param {Object} options
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.setOptions = function(options, overwrite) {
    // overwrite is only for internal use
    if (overwrite) {
      // ensure that _mongooseOptions & options are two different objects
      this._mongooseOptions = (options && utils.clone(options)) || {};
      this.options = options || {};
  
      if ('populate' in options) {
        this.populate(this._mongooseOptions);
      }
      return this;
    }
    if (options == null) {
      return this;
    }
    if (typeof options !== 'object') {
      throw new Error('Options must be an object, got "' + options + '"');
    }
  
    if (Array.isArray(options.populate)) {
      const populate = options.populate;
      delete options.populate;
      const _numPopulate = populate.length;
      for (let i = 0; i < _numPopulate; ++i) {
        this.populate(populate[i]);
      }
    }
  
    if ('useFindAndModify' in options) {
      this._mongooseOptions.useFindAndModify = options.useFindAndModify;
      delete options.useFindAndModify;
    }
    if ('omitUndefined' in options) {
      this._mongooseOptions.omitUndefined = options.omitUndefined;
      delete options.omitUndefined;
    }
    if ('setDefaultsOnInsert' in options) {
      this._mongooseOptions.setDefaultsOnInsert = options.setDefaultsOnInsert;
      delete options.setDefaultsOnInsert;
    }
    if ('overwriteDiscriminatorKey' in options) {
      this._mongooseOptions.overwriteDiscriminatorKey = options.overwriteDiscriminatorKey;
      delete options.overwriteDiscriminatorKey;
    }
  
    return Query.base.setOptions.call(this, options);
  };
  
  /**
   * Sets the [`explain` option](https://docs.mongodb.com/manual/reference/method/cursor.explain/),
   * which makes this query return detailed execution stats instead of the actual
   * query result. This method is useful for determining what index your queries
   * use.
   *
   * Calling `query.explain(v)` is equivalent to `query.setOptions({ explain: v })`
   *
   * ####Example:
   *
   *     const query = new Query();
   *     const res = await query.find({ a: 1 }).explain('queryPlanner');
   *     console.log(res);
   *
   * @param {String} [verbose] The verbosity mode. Either 'queryPlanner', 'executionStats', or 'allPlansExecution'. The default is 'queryPlanner'
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.explain = function(verbose) {
    if (arguments.length === 0) {
      this.options.explain = true;
    } else if (verbose === false) {
      delete this.options.explain;
    } else {
      this.options.explain = verbose;
    }
    return this;
  };
  
  /**
   * Sets the [`allowDiskUse` option](https://docs.mongodb.com/manual/reference/method/cursor.allowDiskUse/),
   * which allows the MongoDB server to use more than 100 MB for this query's `sort()`. This option can
   * let you work around `QueryExceededMemoryLimitNoDiskUseAllowed` errors from the MongoDB server.
   *
   * Note that this option requires MongoDB server >= 4.4. Setting this option is a no-op for MongoDB 4.2
   * and earlier.
   *
   * Calling `query.allowDiskUse(v)` is equivalent to `query.setOptions({ allowDiskUse: v })`
   *
   * ####Example:
   *
   *     await query.find().sort({ name: 1 }).allowDiskUse(true);
   *     // Equivalent:
   *     await query.find().sort({ name: 1 }).allowDiskUse();
   *
   * @param {Boolean} [v] Enable/disable `allowDiskUse`. If called with 0 arguments, sets `allowDiskUse: true`
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.allowDiskUse = function(v) {
    if (arguments.length === 0) {
      this.options.allowDiskUse = true;
    } else if (v === false) {
      delete this.options.allowDiskUse;
    } else {
      this.options.allowDiskUse = v;
    }
    return this;
  };
  
  /**
   * Sets the [maxTimeMS](https://docs.mongodb.com/manual/reference/method/cursor.maxTimeMS/)
   * option. This will tell the MongoDB server to abort if the query or write op
   * has been running for more than `ms` milliseconds.
   *
   * Calling `query.maxTimeMS(v)` is equivalent to `query.setOptions({ maxTimeMS: v })`
   *
   * ####Example:
   *
   *     const query = new Query();
   *     // Throws an error 'operation exceeded time limit' as long as there's
   *     // >= 1 doc in the queried collection
   *     const res = await query.find({ $where: 'sleep(1000) || true' }).maxTimeMS(100);
   *
   * @param {Number} [ms] The number of milliseconds
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.maxTimeMS = function(ms) {
    this.options.maxTimeMS = ms;
    return this;
  };
  
  /**
   * Returns the current query filter (also known as conditions) as a [POJO](https://masteringjs.io/tutorials/fundamentals/pojo).
   *
   * ####Example:
   *
   *     const query = new Query();
   *     query.find({ a: 1 }).where('b').gt(2);
   *     query.getFilter(); // { a: 1, b: { $gt: 2 } }
   *
   * @return {Object} current query filter
   * @api public
   */
  
  Query.prototype.getFilter = function() {
    return this._conditions;
  };
  
  /**
   * Returns the current query filter. Equivalent to `getFilter()`.
   *
   * You should use `getFilter()` instead of `getQuery()` where possible. `getQuery()`
   * will likely be deprecated in a future release.
   *
   * ####Example:
   *
   *     const query = new Query();
   *     query.find({ a: 1 }).where('b').gt(2);
   *     query.getQuery(); // { a: 1, b: { $gt: 2 } }
   *
   * @return {Object} current query filter
   * @api public
   */
  
  Query.prototype.getQuery = function() {
    return this._conditions;
  };
  
  /**
   * Sets the query conditions to the provided JSON object.
   *
   * ####Example:
   *
   *     const query = new Query();
   *     query.find({ a: 1 })
   *     query.setQuery({ a: 2 });
   *     query.getQuery(); // { a: 2 }
   *
   * @param {Object} new query conditions
   * @return {undefined}
   * @api public
   */
  
  Query.prototype.setQuery = function(val) {
    this._conditions = val;
  };
  
  /**
   * Returns the current update operations as a JSON object.
   *
   * ####Example:
   *
   *     const query = new Query();
   *     query.update({}, { $set: { a: 5 } });
   *     query.getUpdate(); // { $set: { a: 5 } }
   *
   * @return {Object} current update operations
   * @api public
   */
  
  Query.prototype.getUpdate = function() {
    return this._update;
  };
  
  /**
   * Sets the current update operation to new value.
   *
   * ####Example:
   *
   *     const query = new Query();
   *     query.update({}, { $set: { a: 5 } });
   *     query.setUpdate({ $set: { b: 6 } });
   *     query.getUpdate(); // { $set: { b: 6 } }
   *
   * @param {Object} new update operation
   * @return {undefined}
   * @api public
   */
  
  Query.prototype.setUpdate = function(val) {
    this._update = val;
  };
  
  /**
   * Returns fields selection for this query.
   *
   * @method _fieldsForExec
   * @return {Object}
   * @api private
   * @receiver Query
   */
  
  Query.prototype._fieldsForExec = function() {
    return utils.clone(this._fields);
  };
  
  
  /**
   * Return an update document with corrected `$set` operations.
   *
   * @method _updateForExec
   * @api private
   * @receiver Query
   */
  
  Query.prototype._updateForExec = function() {
    const update = utils.clone(this._update, {
      transform: false,
      depopulate: true
    });
    const ops = Object.keys(update);
    let i = ops.length;
    const ret = {};
  
    while (i--) {
      const op = ops[i];
  
      if (this.options.overwrite) {
        ret[op] = update[op];
        continue;
      }
  
      if ('$' !== op[0]) {
        // fix up $set sugar
        if (!ret.$set) {
          if (update.$set) {
            ret.$set = update.$set;
          } else {
            ret.$set = {};
          }
        }
        ret.$set[op] = update[op];
        ops.splice(i, 1);
        if (!~ops.indexOf('$set')) ops.push('$set');
      } else if ('$set' === op) {
        if (!ret.$set) {
          ret[op] = update[op];
        }
      } else {
        ret[op] = update[op];
      }
    }
  
    return ret;
  };
  
  /**
   * Makes sure _path is set.
   *
   * @method _ensurePath
   * @param {String} method
   * @api private
   * @receiver Query
   */
  
  /**
   * Determines if `conds` can be merged using `mquery().merge()`
   *
   * @method canMerge
   * @memberOf Query
   * @instance
   * @param {Object} conds
   * @return {Boolean}
   * @api private
   */
  
  /**
   * Returns default options for this query.
   *
   * @param {Model} model
   * @api private
   */
  
  Query.prototype._optionsForExec = function(model) {
    const options = utils.clone(this.options);
    delete options.populate;
    model = model || this.model;
  
    if (!model) {
      return options;
    }
  
    const safe = get(model, 'schema.options.safe', null);
    if (!('safe' in options) && safe != null) {
      setSafe(options, safe);
    }
  
    // Apply schema-level `writeConcern` option
    applyWriteConcern(model.schema, options);
  
    const readPreference = get(model, 'schema.options.read');
    if (!('readPreference' in options) && readPreference) {
      options.readPreference = readPreference;
    }
  
    if (options.upsert !== void 0) {
      options.upsert = !!options.upsert;
    }
    if (options.writeConcern) {
      if (options.j) {
        options.writeConcern.j = options.j;
        delete options.j;
      }
      if (options.w) {
        options.writeConcern.w = options.w;
        delete options.w;
      }
      if (options.wtimeout) {
        options.writeConcern.wtimeout = options.wtimeout;
        delete options.wtimeout;
      }
    }
    return options;
  };
  
  /*!
   * ignore
   */
  
  const safeDeprecationWarning = 'Mongoose: the `safe` option is deprecated. ' +
    'Use write concerns instead: http://bit.ly/mongoose-w';
  
  const setSafe = util.deprecate(function setSafe(options, safe) {
    options.safe = safe;
  }, safeDeprecationWarning);
  
  /**
   * Sets the lean option.
   *
   * Documents returned from queries with the `lean` option enabled are plain
   * javascript objects, not [Mongoose Documents](/api/document.html). They have no
   * `save` method, getters/setters, virtuals, or other Mongoose features.
   *
   * ####Example:
   *
   *     new Query().lean() // true
   *     new Query().lean(true)
   *     new Query().lean(false)
   *
   *     const docs = await Model.find().lean();
   *     docs[0] instanceof mongoose.Document; // false
   *
   * [Lean is great for high-performance, read-only cases](/docs/tutorials/lean.html),
   * especially when combined
   * with [cursors](/docs/queries.html#streaming).
   *
   * If you need virtuals, getters/setters, or defaults with `lean()`, you need
   * to use a plugin. See:
   *
   * - [mongoose-lean-virtuals](https://plugins.mongoosejs.io/plugins/lean-virtuals)
   * - [mongoose-lean-getters](https://plugins.mongoosejs.io/plugins/lean-getters)
   * - [mongoose-lean-defaults](https://www.npmjs.com/package/mongoose-lean-defaults)
   *
   * @param {Boolean|Object} bool defaults to true
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.lean = function(v) {
    this._mongooseOptions.lean = arguments.length ? v : true;
    return this;
  };
  
  /**
   * Adds a `$set` to this query's update without changing the operation.
   * This is useful for query middleware so you can add an update regardless
   * of whether you use `updateOne()`, `updateMany()`, `findOneAndUpdate()`, etc.
   *
   * ####Example:
   *
   *     // Updates `{ $set: { updatedAt: new Date() } }`
   *     new Query().updateOne({}, {}).set('updatedAt', new Date());
   *     new Query().updateMany({}, {}).set({ updatedAt: new Date() });
   *
   * @param {String|Object} path path or object of key/value pairs to set
   * @param {Any} [val] the value to set
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.set = function(path, val) {
    if (typeof path === 'object') {
      const keys = Object.keys(path);
      for (const key of keys) {
        this.set(key, path[key]);
      }
      return this;
    }
  
    this._update = this._update || {};
    this._update.$set = this._update.$set || {};
    this._update.$set[path] = val;
    return this;
  };
  
  /**
   * For update operations, returns the value of a path in the update's `$set`.
   * Useful for writing getters/setters that can work with both update operations
   * and `save()`.
   *
   * ####Example:
   *
   *     const query = Model.updateOne({}, { $set: { name: 'Jean-Luc Picard' } });
   *     query.get('name'); // 'Jean-Luc Picard'
   *
   * @param {String|Object} path path or object of key/value pairs to get
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.get = function get(path) {
    const update = this._update;
    if (update == null) {
      return void 0;
    }
    const $set = update.$set;
    if ($set == null) {
      return update[path];
    }
  
    if (utils.hasUserDefinedProperty(update, path)) {
      return update[path];
    }
    if (utils.hasUserDefinedProperty($set, path)) {
      return $set[path];
    }
  
    return void 0;
  };
  
  /**
   * Gets/sets the error flag on this query. If this flag is not null or
   * undefined, the `exec()` promise will reject without executing.
   *
   * ####Example:
   *
   *     Query().error(); // Get current error value
   *     Query().error(null); // Unset the current error
   *     Query().error(new Error('test')); // `exec()` will resolve with test
   *     Schema.pre('find', function() {
   *       if (!this.getQuery().userId) {
   *         this.error(new Error('Not allowed to query without setting userId'));
   *       }
   *     });
   *
   * Note that query casting runs **after** hooks, so cast errors will override
   * custom errors.
   *
   * ####Example:
   *     const TestSchema = new Schema({ num: Number });
   *     const TestModel = db.model('Test', TestSchema);
   *     TestModel.find({ num: 'not a number' }).error(new Error('woops')).exec(function(error) {
   *       // `error` will be a cast error because `num` failed to cast
   *     });
   *
   * @param {Error|null} err if set, `exec()` will fail fast before sending the query to MongoDB
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.error = function error(err) {
    if (arguments.length === 0) {
      return this._error;
    }
  
    this._error = err;
    return this;
  };
  
  /*!
   * ignore
   */
  
  Query.prototype._unsetCastError = function _unsetCastError() {
    if (this._error != null && !(this._error instanceof CastError)) {
      return;
    }
    return this.error(null);
  };
  
  /**
   * Getter/setter around the current mongoose-specific options for this query
   * Below are the current Mongoose-specific options.
   *
   * - `populate`: an array representing what paths will be populated. Should have one entry for each call to [`Query.prototype.populate()`](/docs/api.html#query_Query-populate)
   * - `lean`: if truthy, Mongoose will not [hydrate](/docs/api.html#model_Model.hydrate) any documents that are returned from this query. See [`Query.prototype.lean()`](/docs/api.html#query_Query-lean) for more information.
   * - `strict`: controls how Mongoose handles keys that aren't in the schema for updates. This option is `true` by default, which means Mongoose will silently strip any paths in the update that aren't in the schema. See the [`strict` mode docs](/docs/guide.html#strict) for more information.
   * - `strictQuery`: controls how Mongoose handles keys that aren't in the schema for the query `filter`. This option is `false` by default for backwards compatibility, which means Mongoose will allow `Model.find({ foo: 'bar' })` even if `foo` is not in the schema. See the [`strictQuery` docs](/docs/guide.html#strictQuery) for more information.
   * - `useFindAndModify`: used to work around the [`findAndModify()` deprecation warning](/docs/deprecations.html#findandmodify)
   * - `omitUndefined`: delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
   * - `nearSphere`: use `$nearSphere` instead of `near()`. See the [`Query.prototype.nearSphere()` docs](/docs/api.html#query_Query-nearSphere)
   *
   * Mongoose maintains a separate object for internal options because
   * Mongoose sends `Query.prototype.options` to the MongoDB server, and the
   * above options are not relevant for the MongoDB server.
   *
   * @param {Object} options if specified, overwrites the current options
   * @return {Object} the options
   * @api public
   */
  
  Query.prototype.mongooseOptions = function(v) {
    if (arguments.length > 0) {
      this._mongooseOptions = v;
    }
    return this._mongooseOptions;
  };
  
  /*!
   * ignore
   */
  
  Query.prototype._castConditions = function() {
    try {
      this.cast(this.model);
      this._unsetCastError();
    } catch (err) {
      this.error(err);
    }
  };
  
  /*!
   * ignore
   */
  
  function _castArrayFilters(query) {
    try {
      castArrayFilters(query);
    } catch (err) {
      query.error(err);
    }
  }
  
  /**
   * Thunk around find()
   *
   * @param {Function} [callback]
   * @return {Query} this
   * @api private
   */
  Query.prototype._find = wrapThunk(function(callback) {
    this._castConditions();
  
    if (this.error() != null) {
      callback(this.error());
      return null;
    }
  
    callback = _wrapThunkCallback(this, callback);
  
    this._applyPaths();
    this._fields = this._castFields(this._fields);
  
    const fields = this._fieldsForExec();
    const mongooseOptions = this._mongooseOptions;
    const _this = this;
    const userProvidedFields = _this._userProvidedFields || {};
  
    applyGlobalMaxTimeMS(this.options, this.model);
  
    // Separate options to pass down to `completeMany()` in case we need to
    // set a session on the document
    const completeManyOptions = Object.assign({}, {
      session: get(this, 'options.session', null)
    });
  
    const cb = (err, docs) => {
      if (err) {
        return callback(err);
      }
  
      if (docs.length === 0) {
        return callback(null, docs);
      }
      if (this.options.explain) {
        return callback(null, docs);
      }
  
      if (!mongooseOptions.populate) {
        return mongooseOptions.lean ?
          callback(null, docs) :
          completeMany(_this.model, docs, fields, userProvidedFields, completeManyOptions, callback);
      }
  
      const pop = helpers.preparePopulationOptionsMQ(_this, mongooseOptions);
      completeManyOptions.populated = pop;
      _this.model.populate(docs, pop, function(err, docs) {
        if (err) return callback(err);
        return mongooseOptions.lean ?
          callback(null, docs) :
          completeMany(_this.model, docs, fields, userProvidedFields, completeManyOptions, callback);
      });
    };
  
    const options = this._optionsForExec();
    options.projection = this._fieldsForExec();
    const filter = this._conditions;
  
    this._collection.find(filter, options, cb);
    return null;
  });
  
  /**
   * Find all documents that match `selector`. The result will be an array of documents.
   *
   * If there are too many documents in the result to fit in memory, use
   * [`Query.prototype.cursor()`](api.html#query_Query-cursor)
   *
   * ####Example
   *
   *     // Using async/await
   *     const arr = await Movie.find({ year: { $gte: 1980, $lte: 1989 } });
   *
   *     // Using callbacks
   *     Movie.find({ year: { $gte: 1980, $lte: 1989 } }, function(err, arr) {});
   *
   * @param {Object|ObjectId} [filter] mongodb selector. If not specified, returns all documents.
   * @param {Function} [callback]
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.find = function(conditions, callback) {
    this.op = 'find';
  
    if (typeof conditions === 'function') {
      callback = conditions;
      conditions = {};
    }
  
    conditions = utils.toObject(conditions);
  
    if (mquery.canMerge(conditions)) {
      this.merge(conditions);
  
      prepareDiscriminatorCriteria(this);
    } else if (conditions != null) {
      this.error(new ObjectParameterError(conditions, 'filter', 'find'));
    }
  
    // if we don't have a callback, then just return the query object
    if (!callback) {
      return Query.base.find.call(this);
    }
  
    this.exec(callback);
  
    return this;
  };
  
  /**
   * Merges another Query or conditions object into this one.
   *
   * When a Query is passed, conditions, field selection and options are merged.
   *
   * @param {Query|Object} source
   * @return {Query} this
   */
  
  Query.prototype.merge = function(source) {
    if (!source) {
      return this;
    }
  
    const opts = { overwrite: true };
  
    if (source instanceof Query) {
      // if source has a feature, apply it to ourselves
  
      if (source._conditions) {
        utils.merge(this._conditions, source._conditions, opts);
      }
  
      if (source._fields) {
        this._fields || (this._fields = {});
        utils.merge(this._fields, source._fields, opts);
      }
  
      if (source.options) {
        this.options || (this.options = {});
        utils.merge(this.options, source.options, opts);
      }
  
      if (source._update) {
        this._update || (this._update = {});
        utils.mergeClone(this._update, source._update);
      }
  
      if (source._distinct) {
        this._distinct = source._distinct;
      }
  
      utils.merge(this._mongooseOptions, source._mongooseOptions);
  
      return this;
    }
  
    // plain object
    utils.merge(this._conditions, source, opts);
  
    return this;
  };
  
  /**
   * Adds a collation to this op (MongoDB 3.4 and up)
   *
   * @param {Object} value
   * @return {Query} this
   * @see MongoDB docs https://docs.mongodb.com/manual/reference/method/cursor.collation/#cursor.collation
   * @api public
   */
  
  Query.prototype.collation = function(value) {
    if (this.options == null) {
      this.options = {};
    }
    this.options.collation = value;
    return this;
  };
  
  /**
   * Hydrate a single doc from `findOne()`, `findOneAndUpdate()`, etc.
   *
   * @api private
   */
  
  Query.prototype._completeOne = function(doc, res, callback) {
    if (!doc && !this.options.rawResult) {
      return callback(null, null);
    }
  
    const model = this.model;
    const projection = utils.clone(this._fields);
    const userProvidedFields = this._userProvidedFields || {};
    // `populate`, `lean`
    const mongooseOptions = this._mongooseOptions;
    // `rawResult`
    const options = this.options;
  
    if (options.explain) {
      return callback(null, doc);
    }
  
    if (!mongooseOptions.populate) {
      return mongooseOptions.lean ?
        _completeOneLean(doc, res, options, callback) :
        completeOne(model, doc, res, options, projection, userProvidedFields,
          null, callback);
    }
  
    const pop = helpers.preparePopulationOptionsMQ(this, this._mongooseOptions);
    model.populate(doc, pop, (err, doc) => {
      if (err) {
        return callback(err);
      }
      return mongooseOptions.lean ?
        _completeOneLean(doc, res, options, callback) :
        completeOne(model, doc, res, options, projection, userProvidedFields,
          pop, callback);
    });
  };
  
  /**
   * Thunk around findOne()
   *
   * @param {Function} [callback]
   * @see findOne http://docs.mongodb.org/manual/reference/method/db.collection.findOne/
   * @api private
   */
  
  Query.prototype._findOne = wrapThunk(function(callback) {
    this._castConditions();
  
    if (this.error()) {
      callback(this.error());
      return null;
    }
  
    this._applyPaths();
    this._fields = this._castFields(this._fields);
  
    applyGlobalMaxTimeMS(this.options, this.model);
  
    // don't pass in the conditions because we already merged them in
    Query.base.findOne.call(this, {}, (err, doc) => {
      if (err) {
        callback(err);
        return null;
      }
  
      this._completeOne(doc, null, _wrapThunkCallback(this, callback));
    });
  });
  
  /**
   * Declares the query a findOne operation. When executed, the first found document is passed to the callback.
   *
   * Passing a `callback` executes the query. The result of the query is a single document.
   *
   * * *Note:* `conditions` is optional, and if `conditions` is null or undefined,
   * mongoose will send an empty `findOne` command to MongoDB, which will return
   * an arbitrary document. If you're querying by `_id`, use `Model.findById()`
   * instead.
   *
   * This function triggers the following middleware.
   *
   * - `findOne()`
   *
   * ####Example
   *
   *     const query  = Kitten.where({ color: 'white' });
   *     query.findOne(function (err, kitten) {
   *       if (err) return handleError(err);
   *       if (kitten) {
   *         // doc may be null if no document matched
   *       }
   *     });
   *
   * @param {Object} [filter] mongodb selector
   * @param {Object} [projection] optional fields to return
   * @param {Object} [options] see [`setOptions()`](http://mongoosejs.com/docs/api.html#query_Query-setOptions)
   * @param {Function} [callback] optional params are (error, document)
   * @return {Query} this
   * @see findOne http://docs.mongodb.org/manual/reference/method/db.collection.findOne/
   * @see Query.select #query_Query-select
   * @api public
   */
  
  Query.prototype.findOne = function(conditions, projection, options, callback) {
    this.op = 'findOne';
  
    if (typeof conditions === 'function') {
      callback = conditions;
      conditions = null;
      projection = null;
      options = null;
    } else if (typeof projection === 'function') {
      callback = projection;
      options = null;
      projection = null;
    } else if (typeof options === 'function') {
      callback = options;
      options = null;
    }
  
    // make sure we don't send in the whole Document to merge()
    conditions = utils.toObject(conditions);
  
    if (options) {
      this.setOptions(options);
    }
  
    if (projection) {
      this.select(projection);
    }
  
    if (mquery.canMerge(conditions)) {
      this.merge(conditions);
  
      prepareDiscriminatorCriteria(this);
    } else if (conditions != null) {
      this.error(new ObjectParameterError(conditions, 'filter', 'findOne'));
    }
  
    if (!callback) {
      // already merged in the conditions, don't need to send them in.
      return Query.base.findOne.call(this);
    }
  
    this.exec(callback);
  
    return this;
  };
  
  /**
   * Thunk around count()
   *
   * @param {Function} [callback]
   * @see count http://docs.mongodb.org/manual/reference/method/db.collection.count/
   * @api private
   */
  
  Query.prototype._count = wrapThunk(function(callback) {
    try {
      this.cast(this.model);
    } catch (err) {
      this.error(err);
    }
  
    if (this.error()) {
      return callback(this.error());
    }
  
    applyGlobalMaxTimeMS(this.options, this.model);
  
    const conds = this._conditions;
    const options = this._optionsForExec();
  
    this._collection.count(conds, options, utils.tick(callback));
  });
  
  /**
   * Thunk around countDocuments()
   *
   * @param {Function} [callback]
   * @see countDocuments http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#countDocuments
   * @api private
   */
  
  Query.prototype._countDocuments = wrapThunk(function(callback) {
    try {
      this.cast(this.model);
    } catch (err) {
      this.error(err);
    }
  
    if (this.error()) {
      return callback(this.error());
    }
  
    applyGlobalMaxTimeMS(this.options, this.model);
  
    const conds = this._conditions;
    const options = this._optionsForExec();
  
    this._collection.collection.countDocuments(conds, options, utils.tick(callback));
  });
  
  /**
   * Thunk around estimatedDocumentCount()
   *
   * @param {Function} [callback]
   * @see estimatedDocumentCount http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#estimatedDocumentCount
   * @api private
   */
  
  Query.prototype._estimatedDocumentCount = wrapThunk(function(callback) {
    if (this.error()) {
      return callback(this.error());
    }
  
    const options = this._optionsForExec();
  
    this._collection.collection.estimatedDocumentCount(options, utils.tick(callback));
  });
  
  /**
   * Specifies this query as a `count` query.
   *
   * This method is deprecated. If you want to count the number of documents in
   * a collection, e.g. `count({})`, use the [`estimatedDocumentCount()` function](/docs/api.html#query_Query-estimatedDocumentCount)
   * instead. Otherwise, use the [`countDocuments()`](/docs/api.html#query_Query-countDocuments) function instead.
   *
   * Passing a `callback` executes the query.
   *
   * This function triggers the following middleware.
   *
   * - `count()`
   *
   * ####Example:
   *
   *     const countQuery = model.where({ 'color': 'black' }).count();
   *
   *     query.count({ color: 'black' }).count(callback)
   *
   *     query.count({ color: 'black' }, callback)
   *
   *     query.where('color', 'black').count(function (err, count) {
   *       if (err) return handleError(err);
   *       console.log('there are %d kittens', count);
   *     })
   *
   * @deprecated
   * @param {Object} [filter] count documents that match this object
   * @param {Function} [callback] optional params are (error, count)
   * @return {Query} this
   * @see count http://docs.mongodb.org/manual/reference/method/db.collection.count/
   * @api public
   */
  
  Query.prototype.count = function(filter, callback) {
    this.op = 'count';
    if (typeof filter === 'function') {
      callback = filter;
      filter = undefined;
    }
  
    filter = utils.toObject(filter);
  
    if (mquery.canMerge(filter)) {
      this.merge(filter);
    }
  
    if (!callback) {
      return this;
    }
  
    this.exec(callback);
  
    return this;
  };
  
  /**
   * Specifies this query as a `estimatedDocumentCount()` query. Faster than
   * using `countDocuments()` for large collections because
   * `estimatedDocumentCount()` uses collection metadata rather than scanning
   * the entire collection.
   *
   * `estimatedDocumentCount()` does **not** accept a filter. `Model.find({ foo: bar }).estimatedDocumentCount()`
   * is equivalent to `Model.find().estimatedDocumentCount()`
   *
   * This function triggers the following middleware.
   *
   * - `estimatedDocumentCount()`
   *
   * ####Example:
   *
   *     await Model.find().estimatedDocumentCount();
   *
   * @param {Object} [options] passed transparently to the [MongoDB driver](http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#estimatedDocumentCount)
   * @param {Function} [callback] optional params are (error, count)
   * @return {Query} this
   * @see estimatedDocumentCount http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#estimatedDocumentCount
   * @api public
   */
  
  Query.prototype.estimatedDocumentCount = function(options, callback) {
    this.op = 'estimatedDocumentCount';
    if (typeof options === 'function') {
      callback = options;
      options = undefined;
    }
  
    if (typeof options === 'object' && options != null) {
      this.setOptions(options);
    }
  
    if (!callback) {
      return this;
    }
  
    this.exec(callback);
  
    return this;
  };
  
  /**
   * Specifies this query as a `countDocuments()` query. Behaves like `count()`,
   * except it always does a full collection scan when passed an empty filter `{}`.
   *
   * There are also minor differences in how `countDocuments()` handles
   * [`$where` and a couple geospatial operators](http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#countDocuments).
   * versus `count()`.
   *
   * Passing a `callback` executes the query.
   *
   * This function triggers the following middleware.
   *
   * - `countDocuments()`
   *
   * ####Example:
   *
   *     const countQuery = model.where({ 'color': 'black' }).countDocuments();
   *
   *     query.countDocuments({ color: 'black' }).count(callback);
   *
   *     query.countDocuments({ color: 'black' }, callback);
   *
   *     query.where('color', 'black').countDocuments(function(err, count) {
   *       if (err) return handleError(err);
   *       console.log('there are %d kittens', count);
   *     });
   *
   * The `countDocuments()` function is similar to `count()`, but there are a
   * [few operators that `countDocuments()` does not support](https://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#countDocuments).
   * Below are the operators that `count()` supports but `countDocuments()` does not,
   * and the suggested replacement:
   *
   * - `$where`: [`$expr`](https://docs.mongodb.com/manual/reference/operator/query/expr/)
   * - `$near`: [`$geoWithin`](https://docs.mongodb.com/manual/reference/operator/query/geoWithin/) with [`$center`](https://docs.mongodb.com/manual/reference/operator/query/center/#op._S_center)
   * - `$nearSphere`: [`$geoWithin`](https://docs.mongodb.com/manual/reference/operator/query/geoWithin/) with [`$centerSphere`](https://docs.mongodb.com/manual/reference/operator/query/centerSphere/#op._S_centerSphere)
   *
   * @param {Object} [filter] mongodb selector
   * @param {Function} [callback] optional params are (error, count)
   * @return {Query} this
   * @see countDocuments http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#countDocuments
   * @api public
   */
  
  Query.prototype.countDocuments = function(conditions, callback) {
    this.op = 'countDocuments';
    if (typeof conditions === 'function') {
      callback = conditions;
      conditions = undefined;
    }
  
    conditions = utils.toObject(conditions);
  
    if (mquery.canMerge(conditions)) {
      this.merge(conditions);
    }
  
    if (!callback) {
      return this;
    }
  
    this.exec(callback);
  
    return this;
  };
  
  /**
   * Thunk around distinct()
   *
   * @param {Function} [callback]
   * @see distinct http://docs.mongodb.org/manual/reference/method/db.collection.distinct/
   * @api private
   */
  
  Query.prototype.__distinct = wrapThunk(function __distinct(callback) {
    this._castConditions();
  
    if (this.error()) {
      callback(this.error());
      return null;
    }
  
    applyGlobalMaxTimeMS(this.options, this.model);
  
    const options = this._optionsForExec();
  
    // don't pass in the conditions because we already merged them in
    this._collection.collection.
      distinct(this._distinct, this._conditions, options, callback);
  });
  
  /**
   * Declares or executes a distinct() operation.
   *
   * Passing a `callback` executes the query.
   *
   * This function does not trigger any middleware.
   *
   * ####Example
   *
   *     distinct(field, conditions, callback)
   *     distinct(field, conditions)
   *     distinct(field, callback)
   *     distinct(field)
   *     distinct(callback)
   *     distinct()
   *
   * @param {String} [field]
   * @param {Object|Query} [filter]
   * @param {Function} [callback] optional params are (error, arr)
   * @return {Query} this
   * @see distinct http://docs.mongodb.org/manual/reference/method/db.collection.distinct/
   * @api public
   */
  
  Query.prototype.distinct = function(field, conditions, callback) {
    this.op = 'distinct';
    if (!callback) {
      if (typeof conditions === 'function') {
        callback = conditions;
        conditions = undefined;
      } else if (typeof field === 'function') {
        callback = field;
        field = undefined;
        conditions = undefined;
      }
    }
  
    conditions = utils.toObject(conditions);
  
    if (mquery.canMerge(conditions)) {
      this.merge(conditions);
  
      prepareDiscriminatorCriteria(this);
    } else if (conditions != null) {
      this.error(new ObjectParameterError(conditions, 'filter', 'distinct'));
    }
  
    if (field != null) {
      this._distinct = field;
    }
  
    if (callback != null) {
      this.exec(callback);
    }
  
    return this;
  };
  
  /**
   * Sets the sort order
   *
   * If an object is passed, values allowed are `asc`, `desc`, `ascending`, `descending`, `1`, and `-1`.
   *
   * If a string is passed, it must be a space delimited list of path names. The
   * sort order of each path is ascending unless the path name is prefixed with `-`
   * which will be treated as descending.
   *
   * ####Example
   *
   *     // sort by "field" ascending and "test" descending
   *     query.sort({ field: 'asc', test: -1 });
   *
   *     // equivalent
   *     query.sort('field -test');
   *
   * ####Note
   *
   * Cannot be used with `distinct()`
   *
   * @param {Object|String} arg
   * @return {Query} this
   * @see cursor.sort http://docs.mongodb.org/manual/reference/method/cursor.sort/
   * @api public
   */
  
  Query.prototype.sort = function(arg) {
    if (arguments.length > 1) {
      throw new Error('sort() only takes 1 Argument');
    }
  
    return Query.base.sort.call(this, arg);
  };
  
  /**
   * Declare and/or execute this query as a remove() operation. `remove()` is
   * deprecated, you should use [`deleteOne()`](#query_Query-deleteOne)
   * or [`deleteMany()`](#query_Query-deleteMany) instead.
   *
   * This function does not trigger any middleware
   *
   * ####Example
   *
   *     Character.remove({ name: /Stark/ }, callback);
   *
   * This function calls the MongoDB driver's [`Collection#remove()` function](http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#remove).
   * The returned [promise](https://mongoosejs.com/docs/queries.html) resolves to an
   * object that contains 3 properties:
   *
   * - `ok`: `1` if no errors occurred
   * - `deletedCount`: the number of documents deleted
   * - `n`: the number of documents deleted. Equal to `deletedCount`.
   *
   * ####Example
   *
   *     const res = await Character.remove({ name: /Stark/ });
   *     // Number of docs deleted
   *     res.deletedCount;
   *
   * ####Note
   *
   * Calling `remove()` creates a [Mongoose query](./queries.html), and a query
   * does not execute until you either pass a callback, call [`Query#then()`](#query_Query-then),
   * or call [`Query#exec()`](#query_Query-exec).
   *
   *     // not executed
   *     const query = Character.remove({ name: /Stark/ });
   *
   *     // executed
   *     Character.remove({ name: /Stark/ }, callback);
   *     Character.remove({ name: /Stark/ }).remove(callback);
   *
   *     // executed without a callback
   *     Character.exec();
   *
   * @param {Object|Query} [filter] mongodb selector
   * @param {Function} [callback] optional params are (error, mongooseDeleteResult)
   * @return {Query} this
   * @deprecated
   * @see deleteWriteOpResult http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#~deleteWriteOpResult
   * @see MongoDB driver remove http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#remove
   * @api public
   */
  
  Query.prototype.remove = function(filter, callback) {
    this.op = 'remove';
    if (typeof filter === 'function') {
      callback = filter;
      filter = null;
    }
  
    filter = utils.toObject(filter);
  
    if (mquery.canMerge(filter)) {
      this.merge(filter);
  
      prepareDiscriminatorCriteria(this);
    } else if (filter != null) {
      this.error(new ObjectParameterError(filter, 'filter', 'remove'));
    }
  
    if (!callback) {
      return Query.base.remove.call(this);
    }
  
    this.exec(callback);
    return this;
  };
  
  /*!
   * ignore
   */
  
  Query.prototype._remove = wrapThunk(function(callback) {
    this._castConditions();
  
    if (this.error() != null) {
      callback(this.error());
      return this;
    }
  
    callback = _wrapThunkCallback(this, callback);
  
    return Query.base.remove.call(this, helpers.handleDeleteWriteOpResult(callback));
  });
  
  /**
   * Declare and/or execute this query as a `deleteOne()` operation. Works like
   * remove, except it deletes at most one document regardless of the `single`
   * option.
   *
   * This function triggers `deleteOne` middleware.
   *
   * ####Example
   *
   *     await Character.deleteOne({ name: 'Eddard Stark' });
   *
   *     // Using callbacks:
   *     Character.deleteOne({ name: 'Eddard Stark' }, callback);
   *
   * This function calls the MongoDB driver's [`Collection#deleteOne()` function](http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#deleteOne).
   * The returned [promise](https://mongoosejs.com/docs/queries.html) resolves to an
   * object that contains 3 properties:
   *
   * - `ok`: `1` if no errors occurred
   * - `deletedCount`: the number of documents deleted
   * - `n`: the number of documents deleted. Equal to `deletedCount`.
   *
   * ####Example
   *
   *     const res = await Character.deleteOne({ name: 'Eddard Stark' });
   *     // `1` if MongoDB deleted a doc, `0` if no docs matched the filter `{ name: ... }`
   *     res.deletedCount;
   *
   * @param {Object|Query} [filter] mongodb selector
   * @param {Object} [options] optional see [`Query.prototype.setOptions()`](http://mongoosejs.com/docs/api.html#query_Query-setOptions)
   * @param {Function} [callback] optional params are (error, mongooseDeleteResult)
   * @return {Query} this
   * @see deleteWriteOpResult http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#~deleteWriteOpResult
   * @see MongoDB Driver deleteOne http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#deleteOne
   * @api public
   */
  
  Query.prototype.deleteOne = function(filter, options, callback) {
    this.op = 'deleteOne';
    if (typeof filter === 'function') {
      callback = filter;
      filter = null;
      options = null;
    } else if (typeof options === 'function') {
      callback = options;
      options = null;
    } else {
      this.setOptions(options);
    }
  
    filter = utils.toObject(filter);
  
    if (mquery.canMerge(filter)) {
      this.merge(filter);
  
      prepareDiscriminatorCriteria(this);
    } else if (filter != null) {
      this.error(new ObjectParameterError(filter, 'filter', 'deleteOne'));
    }
  
    if (!callback) {
      return Query.base.deleteOne.call(this);
    }
  
    this.exec.call(this, callback);
  
    return this;
  };
  
  /*!
   * Internal thunk for `deleteOne()`
   */
  
  Query.prototype._deleteOne = wrapThunk(function(callback) {
    this._castConditions();
  
    if (this.error() != null) {
      callback(this.error());
      return this;
    }
  
    callback = _wrapThunkCallback(this, callback);
  
    return Query.base.deleteOne.call(this, helpers.handleDeleteWriteOpResult(callback));
  });
  
  /**
   * Declare and/or execute this query as a `deleteMany()` operation. Works like
   * remove, except it deletes _every_ document that matches `filter` in the
   * collection, regardless of the value of `single`.
   *
   * This function triggers `deleteMany` middleware.
   *
   * ####Example
   *
   *     await Character.deleteMany({ name: /Stark/, age: { $gte: 18 } });
   *
   *     // Using callbacks:
   *     Character.deleteMany({ name: /Stark/, age: { $gte: 18 } }, callback);
   *
   * This function calls the MongoDB driver's [`Collection#deleteMany()` function](http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#deleteMany).
   * The returned [promise](https://mongoosejs.com/docs/queries.html) resolves to an
   * object that contains 3 properties:
   *
   * - `ok`: `1` if no errors occurred
   * - `deletedCount`: the number of documents deleted
   * - `n`: the number of documents deleted. Equal to `deletedCount`.
   *
   * ####Example
   *
   *     const res = await Character.deleteMany({ name: /Stark/, age: { $gte: 18 } });
   *     // `0` if no docs matched the filter, number of docs deleted otherwise
   *     res.deletedCount;
   *
   * @param {Object|Query} [filter] mongodb selector
   * @param {Object} [options] optional see [`Query.prototype.setOptions()`](http://mongoosejs.com/docs/api.html#query_Query-setOptions)
   * @param {Function} [callback] optional params are (error, mongooseDeleteResult)
   * @return {Query} this
   * @see deleteWriteOpResult http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#~deleteWriteOpResult
   * @see MongoDB Driver deleteMany http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#deleteMany
   * @api public
   */
  
  Query.prototype.deleteMany = function(filter, options, callback) {
    this.op = 'deleteMany';
    if (typeof filter === 'function') {
      callback = filter;
      filter = null;
      options = null;
    } else if (typeof options === 'function') {
      callback = options;
      options = null;
    } else {
      this.setOptions(options);
    }
  
    filter = utils.toObject(filter);
  
    if (mquery.canMerge(filter)) {
      this.merge(filter);
  
      prepareDiscriminatorCriteria(this);
    } else if (filter != null) {
      this.error(new ObjectParameterError(filter, 'filter', 'deleteMany'));
    }
  
    if (!callback) {
      return Query.base.deleteMany.call(this);
    }
  
    this.exec.call(this, callback);
  
    return this;
  };
  
  /*!
   * Internal thunk around `deleteMany()`
   */
  
  Query.prototype._deleteMany = wrapThunk(function(callback) {
    this._castConditions();
  
    if (this.error() != null) {
      callback(this.error());
      return this;
    }
  
    callback = _wrapThunkCallback(this, callback);
  
    return Query.base.deleteMany.call(this, helpers.handleDeleteWriteOpResult(callback));
  });
  
  /*!
   * hydrates a document
   *
   * @param {Model} model
   * @param {Document} doc
   * @param {Object} res 3rd parameter to callback
   * @param {Object} fields
   * @param {Query} self
   * @param {Array} [pop] array of paths used in population
   * @param {Function} callback
   */
  
  function completeOne(model, doc, res, options, fields, userProvidedFields, pop, callback) {
    const opts = pop ?
      { populated: pop }
      : undefined;
  
    if (options.rawResult && doc == null) {
      _init(null);
      return null;
    }
  
    const casted = helpers.createModel(model, doc, fields, userProvidedFields);
    try {
      casted.init(doc, opts, _init);
    } catch (error) {
      _init(error);
    }
  
    function _init(err) {
      if (err) {
        return process.nextTick(() => callback(err));
      }
  
  
      if (options.rawResult) {
        if (doc && casted) {
          casted.$session(options.session);
          res.value = casted;
        } else {
          res.value = null;
        }
        return process.nextTick(() => callback(null, res));
      }
      casted.$session(options.session);
      process.nextTick(() => callback(null, casted));
    }
  }
  
  /*!
   * If the model is a discriminator type and not root, then add the key & value to the criteria.
   */
  
  function prepareDiscriminatorCriteria(query) {
    if (!query || !query.model || !query.model.schema) {
      return;
    }
  
    const schema = query.model.schema;
  
    if (schema && schema.discriminatorMapping && !schema.discriminatorMapping.isRoot) {
      query._conditions[schema.discriminatorMapping.key] = schema.discriminatorMapping.value;
    }
  }
  
  /**
   * Issues a mongodb [findAndModify](http://www.mongodb.org/display/DOCS/findAndModify+Command) update command.
   *
   * Finds a matching document, updates it according to the `update` arg, passing any `options`, and returns the found
   * document (if any) to the callback. The query executes if
   * `callback` is passed.
   *
   * This function triggers the following middleware.
   *
   * - `findOneAndUpdate()`
   *
   * ####Available options
   *
   * - `new`: bool - if true, return the modified document rather than the original. defaults to false (changed in 4.0)
   * - `upsert`: bool - creates the object if it doesn't exist. defaults to false.
   * - `fields`: {Object|String} - Field selection. Equivalent to `.select(fields).findOneAndUpdate()`
   * - `sort`: if multiple docs are found by the conditions, sets the sort order to choose which doc to update
   * - `maxTimeMS`: puts a time limit on the query - requires mongodb >= 2.6.0
   * - `runValidators`: if true, runs [update validators](/docs/validation.html#update-validators) on this command. Update validators validate the update operation against the model's schema.
   * - `setDefaultsOnInsert`: if this and `upsert` are true, mongoose will apply the [defaults](http://mongoosejs.com/docs/defaults.html) specified in the model's schema if a new document is created. This option only works on MongoDB >= 2.4 because it relies on [MongoDB's `$setOnInsert` operator](https://docs.mongodb.org/v2.4/reference/operator/update/setOnInsert/).
   * - `rawResult`: if true, returns the [raw result from the MongoDB driver](http://mongodb.github.io/node-mongodb-native/2.0/api/Collection.html#findAndModify)
   * - `context` (string) if set to 'query' and `runValidators` is on, `this` will refer to the query in custom validator functions that update validation runs. Does nothing if `runValidators` is false.
   *
   * ####Callback Signature
   *     function(error, doc) {
   *       // error: any errors that occurred
   *       // doc: the document before updates are applied if `new: false`, or after updates if `new = true`
   *     }
   *
   * ####Examples
   *
   *     query.findOneAndUpdate(conditions, update, options, callback) // executes
   *     query.findOneAndUpdate(conditions, update, options)  // returns Query
   *     query.findOneAndUpdate(conditions, update, callback) // executes
   *     query.findOneAndUpdate(conditions, update)           // returns Query
   *     query.findOneAndUpdate(update, callback)             // returns Query
   *     query.findOneAndUpdate(update)                       // returns Query
   *     query.findOneAndUpdate(callback)                     // executes
   *     query.findOneAndUpdate()                             // returns Query
   *
   * @method findOneAndUpdate
   * @memberOf Query
   * @instance
   * @param {Object|Query} [filter]
   * @param {Object} [doc]
   * @param {Object} [options]
   * @param {Boolean} [options.rawResult] if true, returns the [raw result from the MongoDB driver](http://mongodb.github.io/node-mongodb-native/2.0/api/Collection.html#findAndModify)
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {ClientSession} [options.session=null] The session associated with this query. See [transactions docs](/docs/transactions.html).
   * @param {Boolean} [options.multipleCastError] by default, mongoose only returns the first error that occurred in casting the query. Turn on this option to aggregate all the cast errors.
   * @param {Boolean} [options.new=false] By default, `findOneAndUpdate()` returns the document as it was **before** `update` was applied. If you set `new: true`, `findOneAndUpdate()` will instead give you the object after `update` was applied.
   * @param {Object} [options.lean] if truthy, mongoose will return the document as a plain JavaScript object rather than a mongoose document. See [`Query.lean()`](/docs/api.html#query_Query-lean) and [the Mongoose lean tutorial](/docs/tutorials/lean.html).
   * @param {ClientSession} [options.session=null] The session associated with this query. See [transactions docs](/docs/transactions.html).
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {Boolean} [options.omitUndefined=false] If true, delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
   * @param {Boolean} [options.timestamps=null] If set to `false` and [schema-level timestamps](/docs/guide.html#timestamps) are enabled, skip timestamps for this update. Note that this allows you to overwrite timestamps. Does nothing if schema-level timestamps are not set.
   * @param {Boolean} [options.returnOriginal=null] An alias for the `new` option. `returnOriginal: false` is equivalent to `new: true`.
   * @param {Function} [callback] optional params are (error, doc), _unless_ `rawResult` is used, in which case params are (error, writeOpResult)
   * @see Tutorial /docs/tutorials/findoneandupdate.html
   * @see mongodb http://www.mongodb.org/display/DOCS/findAndModify+Command
   * @see writeOpResult http://mongodb.github.io/node-mongodb-native/2.2/api/Collection.html#~WriteOpResult
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.findOneAndUpdate = function(criteria, doc, options, callback) {
    this.op = 'findOneAndUpdate';
    this._validate();
  
    switch (arguments.length) {
      case 3:
        if (typeof options === 'function') {
          callback = options;
          options = {};
        }
        break;
      case 2:
        if (typeof doc === 'function') {
          callback = doc;
          doc = criteria;
          criteria = undefined;
        }
        options = undefined;
        break;
      case 1:
        if (typeof criteria === 'function') {
          callback = criteria;
          criteria = options = doc = undefined;
        } else {
          doc = criteria;
          criteria = options = undefined;
        }
    }
  
    if (mquery.canMerge(criteria)) {
      this.merge(criteria);
    }
  
    // apply doc
    if (doc) {
      this._mergeUpdate(doc);
    }
  
    options = options ? utils.clone(options) : {};
  
    if (options.projection) {
      this.select(options.projection);
      delete options.projection;
    }
    if (options.fields) {
      this.select(options.fields);
      delete options.fields;
    }
  
  
    const returnOriginal = get(this, 'model.base.options.returnOriginal');
    if (options.returnOriginal == null && returnOriginal != null) {
      options.returnOriginal = returnOriginal;
    }
  
    this.setOptions(options);
  
    if (!callback) {
      return this;
    }
  
    this.exec(callback);
  
    return this;
  };
  
  /*!
   * Thunk around findOneAndUpdate()
   *
   * @param {Function} [callback]
   * @api private
   */
  
  Query.prototype._findOneAndUpdate = wrapThunk(function(callback) {
    if (this.error() != null) {
      return callback(this.error());
    }
  
    this._findAndModify('update', callback);
  });
  
  /**
   * Issues a mongodb [findAndModify](http://www.mongodb.org/display/DOCS/findAndModify+Command) remove command.
   *
   * Finds a matching document, removes it, passing the found document (if any) to
   * the callback. Executes if `callback` is passed.
   *
   * This function triggers the following middleware.
   *
   * - `findOneAndRemove()`
   *
   * ####Available options
   *
   * - `sort`: if multiple docs are found by the conditions, sets the sort order to choose which doc to update
   * - `maxTimeMS`: puts a time limit on the query - requires mongodb >= 2.6.0
   * - `rawResult`: if true, resolves to the [raw result from the MongoDB driver](http://mongodb.github.io/node-mongodb-native/2.0/api/Collection.html#findAndModify)
   *
   * ####Callback Signature
   *     function(error, doc) {
   *       // error: any errors that occurred
   *       // doc: the document before updates are applied if `new: false`, or after updates if `new = true`
   *     }
   *
   * ####Examples
   *
   *     A.where().findOneAndRemove(conditions, options, callback) // executes
   *     A.where().findOneAndRemove(conditions, options)  // return Query
   *     A.where().findOneAndRemove(conditions, callback) // executes
   *     A.where().findOneAndRemove(conditions) // returns Query
   *     A.where().findOneAndRemove(callback)   // executes
   *     A.where().findOneAndRemove()           // returns Query
   *
   * @method findOneAndRemove
   * @memberOf Query
   * @instance
   * @param {Object} [conditions]
   * @param {Object} [options]
   * @param {Boolean} [options.rawResult] if true, returns the [raw result from the MongoDB driver](http://mongodb.github.io/node-mongodb-native/2.0/api/Collection.html#findAndModify)
   * @param {ClientSession} [options.session=null] The session associated with this query. See [transactions docs](/docs/transactions.html).
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {Function} [callback] optional params are (error, document)
   * @return {Query} this
   * @see mongodb http://www.mongodb.org/display/DOCS/findAndModify+Command
   * @api public
   */
  
  Query.prototype.findOneAndRemove = function(conditions, options, callback) {
    this.op = 'findOneAndRemove';
    this._validate();
  
    switch (arguments.length) {
      case 2:
        if (typeof options === 'function') {
          callback = options;
          options = {};
        }
        break;
      case 1:
        if (typeof conditions === 'function') {
          callback = conditions;
          conditions = undefined;
          options = undefined;
        }
        break;
    }
  
    if (mquery.canMerge(conditions)) {
      this.merge(conditions);
    }
  
    options && this.setOptions(options);
  
    if (!callback) {
      return this;
    }
  
    this.exec(callback);
  
    return this;
  };
  
  /**
   * Issues a MongoDB [findOneAndDelete](https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndDelete/) command.
   *
   * Finds a matching document, removes it, and passes the found document (if any)
   * to the callback. Executes if `callback` is passed.
   *
   * This function triggers the following middleware.
   *
   * - `findOneAndDelete()`
   *
   * This function differs slightly from `Model.findOneAndRemove()` in that
   * `findOneAndRemove()` becomes a [MongoDB `findAndModify()` command](https://docs.mongodb.com/manual/reference/method/db.collection.findAndModify/),
   * as opposed to a `findOneAndDelete()` command. For most mongoose use cases,
   * this distinction is purely pedantic. You should use `findOneAndDelete()`
   * unless you have a good reason not to.
   *
   * ####Available options
   *
   * - `sort`: if multiple docs are found by the conditions, sets the sort order to choose which doc to update
   * - `maxTimeMS`: puts a time limit on the query - requires mongodb >= 2.6.0
   * - `rawResult`: if true, resolves to the [raw result from the MongoDB driver](http://mongodb.github.io/node-mongodb-native/2.0/api/Collection.html#findAndModify)
   *
   * ####Callback Signature
   *     function(error, doc) {
   *       // error: any errors that occurred
   *       // doc: the document before updates are applied if `new: false`, or after updates if `new = true`
   *     }
   *
   * ####Examples
   *
   *     A.where().findOneAndDelete(conditions, options, callback) // executes
   *     A.where().findOneAndDelete(conditions, options)  // return Query
   *     A.where().findOneAndDelete(conditions, callback) // executes
   *     A.where().findOneAndDelete(conditions) // returns Query
   *     A.where().findOneAndDelete(callback)   // executes
   *     A.where().findOneAndDelete()           // returns Query
   *
   * @method findOneAndDelete
   * @memberOf Query
   * @param {Object} [conditions]
   * @param {Object} [options]
   * @param {Boolean} [options.rawResult] if true, returns the [raw result from the MongoDB driver](http://mongodb.github.io/node-mongodb-native/2.0/api/Collection.html#findAndModify)
   * @param {ClientSession} [options.session=null] The session associated with this query. See [transactions docs](/docs/transactions.html).
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {Function} [callback] optional params are (error, document)
   * @return {Query} this
   * @see mongodb http://www.mongodb.org/display/DOCS/findAndModify+Command
   * @api public
   */
  
  Query.prototype.findOneAndDelete = function(conditions, options, callback) {
    this.op = 'findOneAndDelete';
    this._validate();
  
    switch (arguments.length) {
      case 2:
        if (typeof options === 'function') {
          callback = options;
          options = {};
        }
        break;
      case 1:
        if (typeof conditions === 'function') {
          callback = conditions;
          conditions = undefined;
          options = undefined;
        }
        break;
    }
  
    if (mquery.canMerge(conditions)) {
      this.merge(conditions);
    }
  
    options && this.setOptions(options);
  
    if (!callback) {
      return this;
    }
  
    this.exec(callback);
  
    return this;
  };
  
  /*!
   * Thunk around findOneAndDelete()
   *
   * @param {Function} [callback]
   * @return {Query} this
   * @api private
   */
  Query.prototype._findOneAndDelete = wrapThunk(function(callback) {
    this._castConditions();
  
    if (this.error() != null) {
      callback(this.error());
      return null;
    }
  
    const filter = this._conditions;
    const options = this._optionsForExec();
    let fields = null;
  
    if (this._fields != null) {
      options.projection = this._castFields(utils.clone(this._fields));
      fields = options.projection;
      if (fields instanceof Error) {
        callback(fields);
        return null;
      }
    }
  
    this._collection.collection.findOneAndDelete(filter, options, _wrapThunkCallback(this, (err, res) => {
      if (err) {
        return callback(err);
      }
  
      const doc = res.value;
  
      return this._completeOne(doc, res, callback);
    }));
  });
  
  /**
   * Issues a MongoDB [findOneAndReplace](https://docs.mongodb.com/manual/reference/method/db.collection.findOneAndReplace/) command.
   *
   * Finds a matching document, removes it, and passes the found document (if any)
   * to the callback. Executes if `callback` is passed.
   *
   * This function triggers the following middleware.
   *
   * - `findOneAndReplace()`
   *
   * ####Available options
   *
   * - `sort`: if multiple docs are found by the conditions, sets the sort order to choose which doc to update
   * - `maxTimeMS`: puts a time limit on the query - requires mongodb >= 2.6.0
   * - `rawResult`: if true, resolves to the [raw result from the MongoDB driver](http://mongodb.github.io/node-mongodb-native/2.0/api/Collection.html#findAndModify)
   *
   * ####Callback Signature
   *     function(error, doc) {
   *       // error: any errors that occurred
   *       // doc: the document before updates are applied if `new: false`, or after updates if `new = true`
   *     }
   *
   * ####Examples
   *
   *     A.where().findOneAndReplace(filter, replacement, options, callback); // executes
   *     A.where().findOneAndReplace(filter, replacement, options); // return Query
   *     A.where().findOneAndReplace(filter, replacement, callback); // executes
   *     A.where().findOneAndReplace(filter); // returns Query
   *     A.where().findOneAndReplace(callback); // executes
   *     A.where().findOneAndReplace(); // returns Query
   *
   * @method findOneAndReplace
   * @memberOf Query
   * @param {Object} [filter]
   * @param {Object} [replacement]
   * @param {Object} [options]
   * @param {Boolean} [options.rawResult] if true, returns the [raw result from the MongoDB driver](http://mongodb.github.io/node-mongodb-native/2.0/api/Collection.html#findAndModify)
   * @param {ClientSession} [options.session=null] The session associated with this query. See [transactions docs](/docs/transactions.html).
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {Boolean} [options.new=false] By default, `findOneAndUpdate()` returns the document as it was **before** `update` was applied. If you set `new: true`, `findOneAndUpdate()` will instead give you the object after `update` was applied.
   * @param {Object} [options.lean] if truthy, mongoose will return the document as a plain JavaScript object rather than a mongoose document. See [`Query.lean()`](/docs/api.html#query_Query-lean) and [the Mongoose lean tutorial](/docs/tutorials/lean.html).
   * @param {ClientSession} [options.session=null] The session associated with this query. See [transactions docs](/docs/transactions.html).
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {Boolean} [options.omitUndefined=false] If true, delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
   * @param {Boolean} [options.timestamps=null] If set to `false` and [schema-level timestamps](/docs/guide.html#timestamps) are enabled, skip timestamps for this update. Note that this allows you to overwrite timestamps. Does nothing if schema-level timestamps are not set.
   * @param {Boolean} [options.returnOriginal=null] An alias for the `new` option. `returnOriginal: false` is equivalent to `new: true`.
   * @param {Function} [callback] optional params are (error, document)
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.findOneAndReplace = function(filter, replacement, options, callback) {
    this.op = 'findOneAndReplace';
    this._validate();
  
    switch (arguments.length) {
      case 3:
        if (typeof options === 'function') {
          callback = options;
          options = void 0;
        }
        break;
      case 2:
        if (typeof replacement === 'function') {
          callback = replacement;
          replacement = void 0;
        }
        break;
      case 1:
        if (typeof filter === 'function') {
          callback = filter;
          filter = void 0;
          replacement = void 0;
          options = void 0;
        }
        break;
    }
  
    if (mquery.canMerge(filter)) {
      this.merge(filter);
    }
  
    if (replacement != null) {
      if (hasDollarKeys(replacement)) {
        throw new Error('The replacement document must not contain atomic operators.');
      }
      this._mergeUpdate(replacement);
    }
  
    options = options || {};
  
    const returnOriginal = get(this, 'model.base.options.returnOriginal');
    if (options.returnOriginal == null && returnOriginal != null) {
      options.returnOriginal = returnOriginal;
    }
  
    this.setOptions(options);
    this.setOptions({ overwrite: true });
  
    if (!callback) {
      return this;
    }
  
    this.exec(callback);
  
    return this;
  };
  
  /*!
   * Thunk around findOneAndReplace()
   *
   * @param {Function} [callback]
   * @return {Query} this
   * @api private
   */
  Query.prototype._findOneAndReplace = wrapThunk(function(callback) {
    this._castConditions();
  
    if (this.error() != null) {
      callback(this.error());
      return null;
    }
  
    const filter = this._conditions;
    const options = this._optionsForExec();
    convertNewToReturnOriginal(options);
    let fields = null;
  
    let castedDoc = new this.model(this._update, null, true);
    this._update = castedDoc;
  
    this._applyPaths();
    if (this._fields != null) {
      options.projection = this._castFields(utils.clone(this._fields));
      fields = options.projection;
      if (fields instanceof Error) {
        callback(fields);
        return null;
      }
    }
  
    castedDoc.validate(err => {
      if (err != null) {
        return callback(err);
      }
  
      if (castedDoc.toBSON) {
        castedDoc = castedDoc.toBSON();
      }
  
      this._collection.collection.findOneAndReplace(filter, castedDoc, options, _wrapThunkCallback(this, (err, res) => {
        if (err) {
          return callback(err);
        }
  
        const doc = res.value;
  
        return this._completeOne(doc, res, callback);
      }));
    });
  });
  
  /*!
   * Support the `new` option as an alternative to `returnOriginal` for backwards
   * compat.
   */
  
  function convertNewToReturnOriginal(options) {
    if ('new' in options) {
      options.returnOriginal = !options['new'];
      delete options['new'];
    }
  }
  
  /*!
   * Thunk around findOneAndRemove()
   *
   * @param {Function} [callback]
   * @return {Query} this
   * @api private
   */
  Query.prototype._findOneAndRemove = wrapThunk(function(callback) {
    if (this.error() != null) {
      callback(this.error());
      return;
    }
  
    this._findAndModify('remove', callback);
  });
  
  /*!
   * Get options from query opts, falling back to the base mongoose object.
   */
  
  function _getOption(query, option, def) {
    const opts = query._optionsForExec(query.model);
  
    if (option in opts) {
      return opts[option];
    }
    if (option in query.model.base.options) {
      return query.model.base.options[option];
    }
    return def;
  }
  
  /*!
   * Override mquery.prototype._findAndModify to provide casting etc.
   *
   * @param {String} type - either "remove" or "update"
   * @param {Function} callback
   * @api private
   */
  
  Query.prototype._findAndModify = function(type, callback) {
    if (typeof callback !== 'function') {
      throw new Error('Expected callback in _findAndModify');
    }
  
    const model = this.model;
    const schema = model.schema;
    const _this = this;
    let fields;
  
    const castedQuery = castQuery(this);
    if (castedQuery instanceof Error) {
      return callback(castedQuery);
    }
  
    _castArrayFilters(this);
  
    const opts = this._optionsForExec(model);
  
    if ('strict' in opts) {
      this._mongooseOptions.strict = opts.strict;
    }
  
    const isOverwriting = this.options.overwrite && !hasDollarKeys(this._update);
    if (isOverwriting) {
      this._update = new this.model(this._update, null, true);
    }
  
    if (type === 'remove') {
      opts.remove = true;
    } else {
      if (!('new' in opts) && !('returnOriginal' in opts)) {
        opts.new = false;
      }
      if (!('upsert' in opts)) {
        opts.upsert = false;
      }
      if (opts.upsert || opts['new']) {
        opts.remove = false;
      }
  
      if (!isOverwriting) {
        this._update = castDoc(this, opts.overwrite);
        const _opts = Object.assign({}, opts, {
          setDefaultsOnInsert: this._mongooseOptions.setDefaultsOnInsert
        });
        this._update = setDefaultsOnInsert(this._conditions, schema, this._update, _opts);
        if (!this._update || Object.keys(this._update).length === 0) {
          if (opts.upsert) {
            // still need to do the upsert to empty doc
            const doc = utils.clone(castedQuery);
            delete doc._id;
            this._update = { $set: doc };
          } else {
            this.findOne(callback);
            return this;
          }
        } else if (this._update instanceof Error) {
          return callback(this._update);
        } else {
          // In order to make MongoDB 2.6 happy (see
          // https://jira.mongodb.org/browse/SERVER-12266 and related issues)
          // if we have an actual update document but $set is empty, junk the $set.
          if (this._update.$set && Object.keys(this._update.$set).length === 0) {
            delete this._update.$set;
          }
        }
      }
  
      if (Array.isArray(opts.arrayFilters)) {
        opts.arrayFilters = removeUnusedArrayFilters(this._update, opts.arrayFilters);
      }
    }
  
    this._applyPaths();
  
    const options = this._mongooseOptions;
  
    if (this._fields) {
      fields = utils.clone(this._fields);
      opts.projection = this._castFields(fields);
      if (opts.projection instanceof Error) {
        return callback(opts.projection);
      }
    }
  
    if (opts.sort) convertSortToArray(opts);
  
    const cb = function(err, doc, res) {
      if (err) {
        return callback(err);
      }
  
      _this._completeOne(doc, res, callback);
    };
  
    let useFindAndModify = true;
    const runValidators = _getOption(this, 'runValidators', false);
    const base = _this.model && _this.model.base;
    const conn = get(model, 'collection.conn', {});
    if ('useFindAndModify' in base.options) {
      useFindAndModify = base.get('useFindAndModify');
    }
    if ('useFindAndModify' in conn.config) {
      useFindAndModify = conn.config.useFindAndModify;
    }
    if ('useFindAndModify' in options) {
      useFindAndModify = options.useFindAndModify;
    }
    if (useFindAndModify === false) {
      // Bypass mquery
      const collection = _this._collection.collection;
      convertNewToReturnOriginal(opts);
  
      if (type === 'remove') {
        collection.findOneAndDelete(castedQuery, opts, _wrapThunkCallback(_this, function(error, res) {
          return cb(error, res ? res.value : res, res);
        }));
  
        return this;
      }
  
      // honors legacy overwrite option for backward compatibility
      const updateMethod = isOverwriting ? 'findOneAndReplace' : 'findOneAndUpdate';
  
      if (runValidators) {
        this.validate(this._update, opts, isOverwriting, error => {
          if (error) {
            return callback(error);
          }
          if (this._update && this._update.toBSON) {
            this._update = this._update.toBSON();
          }
  
          collection[updateMethod](castedQuery, this._update, opts, _wrapThunkCallback(_this, function(error, res) {
            return cb(error, res ? res.value : res, res);
          }));
        });
      } else {
        if (this._update && this._update.toBSON) {
          this._update = this._update.toBSON();
        }
        collection[updateMethod](castedQuery, this._update, opts, _wrapThunkCallback(_this, function(error, res) {
          return cb(error, res ? res.value : res, res);
        }));
      }
  
      return this;
    }
  
    if (runValidators) {
      this.validate(this._update, opts, isOverwriting, function(error) {
        if (error) {
          return callback(error);
        }
        _legacyFindAndModify.call(_this, castedQuery, _this._update, opts, cb);
      });
    } else {
      _legacyFindAndModify.call(_this, castedQuery, _this._update, opts, cb);
    }
  
    return this;
  };
  
  /*!
   * ignore
   */
  
  function _completeOneLean(doc, res, opts, callback) {
    if (opts.rawResult) {
      return callback(null, res);
    }
    return callback(null, doc);
  }
  
  
  /*!
   * ignore
   */
  
  const _legacyFindAndModify = util.deprecate(function(filter, update, opts, cb) {
    if (update && update.toBSON) {
      update = update.toBSON();
    }
    const collection = this._collection;
    const sort = opts != null && Array.isArray(opts.sort) ? opts.sort : [];
    const _cb = _wrapThunkCallback(this, function(error, res) {
      return cb(error, res ? res.value : res, res);
    });
    collection.collection._findAndModify(filter, sort, update, opts, _cb);
  }, 'Mongoose: `findOneAndUpdate()` and `findOneAndDelete()` without the ' +
    '`useFindAndModify` option set to false are deprecated. See: ' +
    'https://mongoosejs.com/docs/deprecations.html#findandmodify');
  
  /*!
   * Override mquery.prototype._mergeUpdate to handle mongoose objects in
   * updates.
   *
   * @param {Object} doc
   * @api private
   */
  
  Query.prototype._mergeUpdate = function(doc) {
    if (doc == null || (typeof doc === 'object' && Object.keys(doc).length === 0)) {
      return;
    }
  
    if (!this._update) {
      this._update = Array.isArray(doc) ? [] : {};
    }
    if (doc instanceof Query) {
      if (Array.isArray(this._update)) {
        throw new Error('Cannot mix array and object updates');
      }
      if (doc._update) {
        utils.mergeClone(this._update, doc._update);
      }
    } else if (Array.isArray(doc)) {
      if (!Array.isArray(this._update)) {
        throw new Error('Cannot mix array and object updates');
      }
      this._update = this._update.concat(doc);
    } else {
      if (Array.isArray(this._update)) {
        throw new Error('Cannot mix array and object updates');
      }
      utils.mergeClone(this._update, doc);
    }
  };
  
  /*!
   * The mongodb driver 1.3.23 only supports the nested array sort
   * syntax. We must convert it or sorting findAndModify will not work.
   */
  
  function convertSortToArray(opts) {
    if (Array.isArray(opts.sort)) {
      return;
    }
    if (!utils.isObject(opts.sort)) {
      return;
    }
  
    const sort = [];
  
    for (const key in opts.sort) {
      if (utils.object.hasOwnProperty(opts.sort, key)) {
        sort.push([key, opts.sort[key]]);
      }
    }
  
    opts.sort = sort;
  }
  
  /*!
   * ignore
   */
  
  function _updateThunk(op, callback) {
    this._castConditions();
  
    _castArrayFilters(this);
  
    if (this.error() != null) {
      callback(this.error());
      return null;
    }
  
    callback = _wrapThunkCallback(this, callback);
    const oldCb = callback;
    callback = function(error, result) {
      oldCb(error, result ? result.result : { ok: 0, n: 0, nModified: 0 });
    };
  
    const castedQuery = this._conditions;
    const options = this._optionsForExec(this.model);
  
    ++this._executionCount;
  
    this._update = utils.clone(this._update, options);
    const isOverwriting = this.options.overwrite && !hasDollarKeys(this._update);
    if (isOverwriting) {
      if (op === 'updateOne' || op === 'updateMany') {
        return callback(new MongooseError('The MongoDB server disallows ' +
          'overwriting documents using `' + op + '`. See: ' +
          'https://mongoosejs.com/docs/deprecations.html#update'));
      }
      this._update = new this.model(this._update, null, true);
    } else {
      this._update = castDoc(this, options.overwrite);
  
      if (this._update instanceof Error) {
        callback(this._update);
        return null;
      }
  
      if (this._update == null || Object.keys(this._update).length === 0) {
        callback(null, 0);
        return null;
      }
  
      const _opts = Object.assign({}, options, {
        setDefaultsOnInsert: this._mongooseOptions.setDefaultsOnInsert
      });
      this._update = setDefaultsOnInsert(this._conditions, this.model.schema,
        this._update, _opts);
    }
  
    if (Array.isArray(options.arrayFilters)) {
      options.arrayFilters = removeUnusedArrayFilters(this._update, options.arrayFilters);
    }
  
    const runValidators = _getOption(this, 'runValidators', false);
    if (runValidators) {
      this.validate(this._update, options, isOverwriting, err => {
        if (err) {
          return callback(err);
        }
  
        if (this._update.toBSON) {
          this._update = this._update.toBSON();
        }
        this._collection[op](castedQuery, this._update, options, callback);
      });
      return null;
    }
  
    if (this._update.toBSON) {
      this._update = this._update.toBSON();
    }
  
    this._collection[op](castedQuery, this._update, options, callback);
    return null;
  }
  
  /*!
   * Mongoose calls this function internally to validate the query if
   * `runValidators` is set
   *
   * @param {Object} castedDoc the update, after casting
   * @param {Object} options the options from `_optionsForExec()`
   * @param {Function} callback
   * @api private
   */
  
  Query.prototype.validate = function validate(castedDoc, options, isOverwriting, callback) {
    return promiseOrCallback(callback, cb => {
      try {
        if (isOverwriting) {
          castedDoc.validate(cb);
        } else {
          updateValidators(this, this.model.schema, castedDoc, options, cb);
        }
      } catch (err) {
        process.nextTick(function() {
          cb(err);
        });
      }
    });
  };
  
  /*!
   * Internal thunk for .update()
   *
   * @param {Function} callback
   * @see Model.update #model_Model.update
   * @api private
   */
  Query.prototype._execUpdate = wrapThunk(function(callback) {
    return _updateThunk.call(this, 'update', callback);
  });
  
  /*!
   * Internal thunk for .updateMany()
   *
   * @param {Function} callback
   * @see Model.update #model_Model.update
   * @api private
   */
  Query.prototype._updateMany = wrapThunk(function(callback) {
    return _updateThunk.call(this, 'updateMany', callback);
  });
  
  /*!
   * Internal thunk for .updateOne()
   *
   * @param {Function} callback
   * @see Model.update #model_Model.update
   * @api private
   */
  Query.prototype._updateOne = wrapThunk(function(callback) {
    return _updateThunk.call(this, 'updateOne', callback);
  });
  
  /*!
   * Internal thunk for .replaceOne()
   *
   * @param {Function} callback
   * @see Model.replaceOne #model_Model.replaceOne
   * @api private
   */
  Query.prototype._replaceOne = wrapThunk(function(callback) {
    return _updateThunk.call(this, 'replaceOne', callback);
  });
  
  /**
   * Declare and/or execute this query as an update() operation.
   *
   * _All paths passed that are not [atomic](https://docs.mongodb.com/manual/tutorial/model-data-for-atomic-operations/#pattern) operations will become `$set` ops._
   *
   * This function triggers the following middleware.
   *
   * - `update()`
   *
   * ####Example
   *
   *     Model.where({ _id: id }).update({ title: 'words' })
   *
   *     // becomes
   *
   *     Model.where({ _id: id }).update({ $set: { title: 'words' }})
   *
   * ####Valid options:
   *
   *  - `upsert` (boolean) whether to create the doc if it doesn't match (false)
   *  - `multi` (boolean) whether multiple documents should be updated (false)
   *  - `runValidators`: if true, runs [update validators](/docs/validation.html#update-validators) on this command. Update validators validate the update operation against the model's schema.
   *  - `setDefaultsOnInsert`: if this and `upsert` are true, mongoose will apply the [defaults](http://mongoosejs.com/docs/defaults.html) specified in the model's schema if a new document is created. This option only works on MongoDB >= 2.4 because it relies on [MongoDB's `$setOnInsert` operator](https://docs.mongodb.org/v2.4/reference/operator/update/setOnInsert/).
   *  - `strict` (boolean) overrides the `strict` option for this update
   *  - `overwrite` (boolean) disables update-only mode, allowing you to overwrite the doc (false)
   *  - `context` (string) if set to 'query' and `runValidators` is on, `this` will refer to the query in custom validator functions that update validation runs. Does nothing if `runValidators` is false.
   *  - `read`
   *  - `writeConcern`
   *
   * ####Note
   *
   * Passing an empty object `{}` as the doc will result in a no-op unless the `overwrite` option is passed. Without the `overwrite` option set, the update operation will be ignored and the callback executed without sending the command to MongoDB so as to prevent accidently overwritting documents in the collection.
   *
   * ####Note
   *
   * The operation is only executed when a callback is passed. To force execution without a callback, we must first call update() and then execute it by using the `exec()` method.
   *
   *     const q = Model.where({ _id: id });
   *     q.update({ $set: { name: 'bob' }}).update(); // not executed
   *
   *     q.update({ $set: { name: 'bob' }}).exec(); // executed
   *
   *     // keys that are not [atomic](https://docs.mongodb.com/manual/tutorial/model-data-for-atomic-operations/#pattern) ops become `$set`.
   *     // this executes the same command as the previous example.
   *     q.update({ name: 'bob' }).exec();
   *
   *     // overwriting with empty docs
   *     const q = Model.where({ _id: id }).setOptions({ overwrite: true })
   *     q.update({ }, callback); // executes
   *
   *     // multi update with overwrite to empty doc
   *     const q = Model.where({ _id: id });
   *     q.setOptions({ multi: true, overwrite: true })
   *     q.update({ });
   *     q.update(callback); // executed
   *
   *     // multi updates
   *     Model.where()
   *          .update({ name: /^match/ }, { $set: { arr: [] }}, { multi: true }, callback)
   *
   *     // more multi updates
   *     Model.where()
   *          .setOptions({ multi: true })
   *          .update({ $set: { arr: [] }}, callback)
   *
   *     // single update by default
   *     Model.where({ email: 'address@example.com' })
   *          .update({ $inc: { counter: 1 }}, callback)
   *
   * API summary
   *
   *     update(filter, doc, options, cb) // executes
   *     update(filter, doc, options)
   *     update(filter, doc, cb) // executes
   *     update(filter, doc)
   *     update(doc, cb) // executes
   *     update(doc)
   *     update(cb) // executes
   *     update(true) // executes
   *     update()
   *
   * @param {Object} [filter]
   * @param {Object} [doc] the update command
   * @param {Object} [options]
   * @param {Boolean} [options.multipleCastError] by default, mongoose only returns the first error that occurred in casting the query. Turn on this option to aggregate all the cast errors.
   * @param {Boolean} [options.omitUndefined=false] If true, delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {Boolean} [options.upsert=false] if true, and no documents found, insert a new document
   * @param {Object} [options.writeConcern=null] sets the [write concern](https://docs.mongodb.com/manual/reference/write-concern/) for replica sets. Overrides the [schema-level write concern](/docs/guide.html#writeConcern)
   * @param {Boolean} [options.timestamps=null] If set to `false` and [schema-level timestamps](/docs/guide.html#timestamps) are enabled, skip timestamps for this update. Does nothing if schema-level timestamps are not set.
   * @param {Function} [callback] params are (error, writeOpResult)
   * @return {Query} this
   * @see Model.update #model_Model.update
   * @see Query docs https://mongoosejs.com/docs/queries.html
   * @see update http://docs.mongodb.org/manual/reference/method/db.collection.update/
   * @see writeOpResult http://mongodb.github.io/node-mongodb-native/2.2/api/Collection.html#~WriteOpResult
   * @see MongoDB docs https://docs.mongodb.com/manual/reference/command/update/#update-command-output
   * @api public
   */
  
  Query.prototype.update = function(conditions, doc, options, callback) {
    if (typeof options === 'function') {
      // .update(conditions, doc, callback)
      callback = options;
      options = null;
    } else if (typeof doc === 'function') {
      // .update(doc, callback);
      callback = doc;
      doc = conditions;
      conditions = {};
      options = null;
    } else if (typeof conditions === 'function') {
      // .update(callback)
      callback = conditions;
      conditions = undefined;
      doc = undefined;
      options = undefined;
    } else if (typeof conditions === 'object' && !doc && !options && !callback) {
      // .update(doc)
      doc = conditions;
      conditions = undefined;
      options = undefined;
      callback = undefined;
    }
  
    return _update(this, 'update', conditions, doc, options, callback);
  };
  
  /**
   * Declare and/or execute this query as an updateMany() operation. Same as
   * `update()`, except MongoDB will update _all_ documents that match
   * `filter` (as opposed to just the first one) regardless of the value of
   * the `multi` option.
   *
   * **Note** updateMany will _not_ fire update middleware. Use `pre('updateMany')`
   * and `post('updateMany')` instead.
   *
   * ####Example:
   *     const res = await Person.updateMany({ name: /Stark$/ }, { isDeleted: true });
   *     res.n; // Number of documents matched
   *     res.nModified; // Number of documents modified
   *
   * This function triggers the following middleware.
   *
   * - `updateMany()`
   *
   * @param {Object} [filter]
   * @param {Object|Array} [update] the update command
   * @param {Object} [options]
   * @param {Boolean} [options.multipleCastError] by default, mongoose only returns the first error that occurred in casting the query. Turn on this option to aggregate all the cast errors.
   * @param {Boolean} [options.omitUndefined=false] If true, delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {Boolean} [options.upsert=false] if true, and no documents found, insert a new document
   * @param {Object} [options.writeConcern=null] sets the [write concern](https://docs.mongodb.com/manual/reference/write-concern/) for replica sets. Overrides the [schema-level write concern](/docs/guide.html#writeConcern)
   * @param {Boolean} [options.timestamps=null] If set to `false` and [schema-level timestamps](/docs/guide.html#timestamps) are enabled, skip timestamps for this update. Does nothing if schema-level timestamps are not set.
   * @param {Function} [callback] params are (error, writeOpResult)
   * @return {Query} this
   * @see Model.update #model_Model.update
   * @see Query docs https://mongoosejs.com/docs/queries.html
   * @see update http://docs.mongodb.org/manual/reference/method/db.collection.update/
   * @see writeOpResult http://mongodb.github.io/node-mongodb-native/2.2/api/Collection.html#~WriteOpResult
   * @see MongoDB docs https://docs.mongodb.com/manual/reference/command/update/#update-command-output
   * @api public
   */
  
  Query.prototype.updateMany = function(conditions, doc, options, callback) {
    if (typeof options === 'function') {
      // .update(conditions, doc, callback)
      callback = options;
      options = null;
    } else if (typeof doc === 'function') {
      // .update(doc, callback);
      callback = doc;
      doc = conditions;
      conditions = {};
      options = null;
    } else if (typeof conditions === 'function') {
      // .update(callback)
      callback = conditions;
      conditions = undefined;
      doc = undefined;
      options = undefined;
    } else if (typeof conditions === 'object' && !doc && !options && !callback) {
      // .update(doc)
      doc = conditions;
      conditions = undefined;
      options = undefined;
      callback = undefined;
    }
  
    return _update(this, 'updateMany', conditions, doc, options, callback);
  };
  
  /**
   * Declare and/or execute this query as an updateOne() operation. Same as
   * `update()`, except it does not support the `multi` or `overwrite` options.
   *
   * - MongoDB will update _only_ the first document that matches `filter` regardless of the value of the `multi` option.
   * - Use `replaceOne()` if you want to overwrite an entire document rather than using [atomic](https://docs.mongodb.com/manual/tutorial/model-data-for-atomic-operations/#pattern) operators like `$set`.
   *
   * **Note** updateOne will _not_ fire update middleware. Use `pre('updateOne')`
   * and `post('updateOne')` instead.
   *
   * ####Example:
   *     const res = await Person.updateOne({ name: 'Jean-Luc Picard' }, { ship: 'USS Enterprise' });
   *     res.n; // Number of documents matched
   *     res.nModified; // Number of documents modified
   *
   * This function triggers the following middleware.
   *
   * - `updateOne()`
   *
   * @param {Object} [filter]
   * @param {Object|Array} [update] the update command
   * @param {Object} [options]
   * @param {Boolean} [options.multipleCastError] by default, mongoose only returns the first error that occurred in casting the query. Turn on this option to aggregate all the cast errors.
   * @param {Boolean} [options.omitUndefined=false] If true, delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {Boolean} [options.upsert=false] if true, and no documents found, insert a new document
   * @param {Object} [options.writeConcern=null] sets the [write concern](https://docs.mongodb.com/manual/reference/write-concern/) for replica sets. Overrides the [schema-level write concern](/docs/guide.html#writeConcern)
   * @param {Boolean} [options.timestamps=null] If set to `false` and [schema-level timestamps](/docs/guide.html#timestamps) are enabled, skip timestamps for this update. Note that this allows you to overwrite timestamps. Does nothing if schema-level timestamps are not set.
   * @param {Function} [callback] params are (error, writeOpResult)
   * @return {Query} this
   * @see Model.update #model_Model.update
   * @see Query docs https://mongoosejs.com/docs/queries.html
   * @see update http://docs.mongodb.org/manual/reference/method/db.collection.update/
   * @see writeOpResult http://mongodb.github.io/node-mongodb-native/2.2/api/Collection.html#~WriteOpResult
   * @see MongoDB docs https://docs.mongodb.com/manual/reference/command/update/#update-command-output
   * @api public
   */
  
  Query.prototype.updateOne = function(conditions, doc, options, callback) {
    if (typeof options === 'function') {
      // .update(conditions, doc, callback)
      callback = options;
      options = null;
    } else if (typeof doc === 'function') {
      // .update(doc, callback);
      callback = doc;
      doc = conditions;
      conditions = {};
      options = null;
    } else if (typeof conditions === 'function') {
      // .update(callback)
      callback = conditions;
      conditions = undefined;
      doc = undefined;
      options = undefined;
    } else if (typeof conditions === 'object' && !doc && !options && !callback) {
      // .update(doc)
      doc = conditions;
      conditions = undefined;
      options = undefined;
      callback = undefined;
    }
  
    return _update(this, 'updateOne', conditions, doc, options, callback);
  };
  
  /**
   * Declare and/or execute this query as a replaceOne() operation. Same as
   * `update()`, except MongoDB will replace the existing document and will
   * not accept any [atomic](https://docs.mongodb.com/manual/tutorial/model-data-for-atomic-operations/#pattern) operators (`$set`, etc.)
   *
   * **Note** replaceOne will _not_ fire update middleware. Use `pre('replaceOne')`
   * and `post('replaceOne')` instead.
   *
   * ####Example:
   *     const res = await Person.replaceOne({ _id: 24601 }, { name: 'Jean Valjean' });
   *     res.n; // Number of documents matched
   *     res.nModified; // Number of documents modified
   *
   * This function triggers the following middleware.
   *
   * - `replaceOne()`
   *
   * @param {Object} [filter]
   * @param {Object} [doc] the update command
   * @param {Object} [options]
   * @param {Boolean} [options.multipleCastError] by default, mongoose only returns the first error that occurred in casting the query. Turn on this option to aggregate all the cast errors.
   * @param {Boolean} [options.omitUndefined=false] If true, delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
   * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
   * @param {Boolean} [options.upsert=false] if true, and no documents found, insert a new document
   * @param {Object} [options.writeConcern=null] sets the [write concern](https://docs.mongodb.com/manual/reference/write-concern/) for replica sets. Overrides the [schema-level write concern](/docs/guide.html#writeConcern)
   * @param {Boolean} [options.timestamps=null] If set to `false` and [schema-level timestamps](/docs/guide.html#timestamps) are enabled, skip timestamps for this update. Does nothing if schema-level timestamps are not set.
   * @param {Function} [callback] params are (error, writeOpResult)
   * @return {Query} this
   * @see Model.update #model_Model.update
   * @see Query docs https://mongoosejs.com/docs/queries.html
   * @see update http://docs.mongodb.org/manual/reference/method/db.collection.update/
   * @see writeOpResult http://mongodb.github.io/node-mongodb-native/2.2/api/Collection.html#~WriteOpResult
   * @see MongoDB docs https://docs.mongodb.com/manual/reference/command/update/#update-command-output
   * @api public
   */
  
  Query.prototype.replaceOne = function(conditions, doc, options, callback) {
    if (typeof options === 'function') {
      // .update(conditions, doc, callback)
      callback = options;
      options = null;
    } else if (typeof doc === 'function') {
      // .update(doc, callback);
      callback = doc;
      doc = conditions;
      conditions = {};
      options = null;
    } else if (typeof conditions === 'function') {
      // .update(callback)
      callback = conditions;
      conditions = undefined;
      doc = undefined;
      options = undefined;
    } else if (typeof conditions === 'object' && !doc && !options && !callback) {
      // .update(doc)
      doc = conditions;
      conditions = undefined;
      options = undefined;
      callback = undefined;
    }
  
    this.setOptions({ overwrite: true });
    return _update(this, 'replaceOne', conditions, doc, options, callback);
  };
  
  /*!
   * Internal helper for update, updateMany, updateOne, replaceOne
   */
  
  function _update(query, op, filter, doc, options, callback) {
    // make sure we don't send in the whole Document to merge()
    query.op = op;
    filter = utils.toObject(filter);
    doc = doc || {};
  
    // strict is an option used in the update checking, make sure it gets set
    if (options != null) {
      if ('strict' in options) {
        query._mongooseOptions.strict = options.strict;
      }
    }
  
    if (!(filter instanceof Query) &&
        filter != null &&
        filter.toString() !== '[object Object]') {
      query.error(new ObjectParameterError(filter, 'filter', op));
    } else {
      query.merge(filter);
    }
  
    if (utils.isObject(options)) {
      query.setOptions(options);
    }
  
    query._mergeUpdate(doc);
  
    // Hooks
    if (callback) {
      query.exec(callback);
  
      return query;
    }
  
    return Query.base[op].call(query, filter, void 0, options, callback);
  }
  
  /**
   * Runs a function `fn` and treats the return value of `fn` as the new value
   * for the query to resolve to.
   *
   * Any functions you pass to `map()` will run **after** any post hooks.
   *
   * ####Example:
   *
   *     const res = await MyModel.findOne().map(res => {
   *       // Sets a `loadedAt` property on the doc that tells you the time the
   *       // document was loaded.
   *       return res == null ?
   *         res :
   *         Object.assign(res, { loadedAt: new Date() });
   *     });
   *
   * @method map
   * @memberOf Query
   * @instance
   * @param {Function} fn function to run to transform the query result
   * @return {Query} this
   */
  
  Query.prototype.map = function(fn) {
    this._transforms.push(fn);
    return this;
  };
  
  /**
   * Make this query throw an error if no documents match the given `filter`.
   * This is handy for integrating with async/await, because `orFail()` saves you
   * an extra `if` statement to check if no document was found.
   *
   * ####Example:
   *
   *     // Throws if no doc returned
   *     await Model.findOne({ foo: 'bar' }).orFail();
   *
   *     // Throws if no document was updated
   *     await Model.updateOne({ foo: 'bar' }, { name: 'test' }).orFail();
   *
   *     // Throws "No docs found!" error if no docs match `{ foo: 'bar' }`
   *     await Model.find({ foo: 'bar' }).orFail(new Error('No docs found!'));
   *
   *     // Throws "Not found" error if no document was found
   *     await Model.findOneAndUpdate({ foo: 'bar' }, { name: 'test' }).
   *       orFail(() => Error('Not found'));
   *
   * @method orFail
   * @memberOf Query
   * @instance
   * @param {Function|Error} [err] optional error to throw if no docs match `filter`. If not specified, `orFail()` will throw a `DocumentNotFoundError`
   * @return {Query} this
   */
  
  Query.prototype.orFail = function(err) {
    this.map(res => {
      switch (this.op) {
        case 'find':
          if (res.length === 0) {
            throw _orFailError(err, this);
          }
          break;
        case 'findOne':
          if (res == null) {
            throw _orFailError(err, this);
          }
          break;
        case 'update':
        case 'updateMany':
        case 'updateOne':
          if (get(res, 'nModified') === 0) {
            throw _orFailError(err, this);
          }
          break;
        case 'findOneAndDelete':
        case 'findOneAndRemove':
          if (get(res, 'lastErrorObject.n') === 0) {
            throw _orFailError(err, this);
          }
          break;
        case 'findOneAndUpdate':
        case 'findOneAndReplace':
          if (get(res, 'lastErrorObject.updatedExisting') === false) {
            throw _orFailError(err, this);
          }
          break;
        case 'deleteMany':
        case 'deleteOne':
        case 'remove':
          if (res.n === 0) {
            throw _orFailError(err, this);
          }
          break;
        default:
          break;
      }
  
      return res;
    });
    return this;
  };
  
  /*!
   * Get the error to throw for `orFail()`
   */
  
  function _orFailError(err, query) {
    if (typeof err === 'function') {
      err = err.call(query);
    }
  
    if (err == null) {
      err = new DocumentNotFoundError(query.getQuery(), query.model.modelName);
    }
  
    return err;
  }
  
  /**
   * Executes the query
   *
   * ####Examples:
   *
   *     const promise = query.exec();
   *     const promise = query.exec('update');
   *
   *     query.exec(callback);
   *     query.exec('find', callback);
   *
   * @param {String|Function} [operation]
   * @param {Function} [callback] optional params depend on the function being called
   * @return {Promise}
   * @api public
   */
  
  Query.prototype.exec = function exec(op, callback) {
    const _this = this;
    // Ensure that `exec()` is the first thing that shows up in
    // the stack when cast errors happen.
    const castError = new CastError();
  
    if (typeof op === 'function') {
      callback = op;
      op = null;
    } else if (typeof op === 'string') {
      this.op = op;
    }
  
    callback = this.model.$handleCallbackError(callback);
  
    return promiseOrCallback(callback, (cb) => {
      cb = this.model.$wrapCallback(cb);
  
      if (!_this.op) {
        cb();
        return;
      }
  
      this._hooks.execPre('exec', this, [], (error) => {
        if (error != null) {
          return cb(_cleanCastErrorStack(castError, error));
        }
        let thunk = '_' + this.op;
        if (this.op === 'update') {
          thunk = '_execUpdate';
        } else if (this.op === 'distinct') {
          thunk = '__distinct';
        }
        this[thunk].call(this, (error, res) => {
          if (error) {
            return cb(_cleanCastErrorStack(castError, error));
          }
  
          this._hooks.execPost('exec', this, [], {}, (error) => {
            if (error) {
              return cb(_cleanCastErrorStack(castError, error));
            }
  
            cb(null, res);
          });
        });
      });
    }, this.model.events);
  };
  
  /*!
   * ignore
   */
  
  function _cleanCastErrorStack(castError, error) {
    if (error instanceof CastError) {
      castError.copy(error);
      return castError;
    }
  
    return error;
  }
  
  /*!
   * ignore
   */
  
  function _wrapThunkCallback(query, cb) {
    return function(error, res) {
      if (error != null) {
        return cb(error);
      }
  
      for (const fn of query._transforms) {
        try {
          res = fn(res);
        } catch (error) {
          return cb(error);
        }
      }
  
      return cb(null, res);
    };
  }
  
  /**
   * Executes the query returning a `Promise` which will be
   * resolved with either the doc(s) or rejected with the error.
   *
   * More about [`then()` in JavaScript](https://masteringjs.io/tutorials/fundamentals/then).
   *
   * @param {Function} [resolve]
   * @param {Function} [reject]
   * @return {Promise}
   * @api public
   */
  
  Query.prototype.then = function(resolve, reject) {
    return this.exec().then(resolve, reject);
  };
  
  /**
   * Executes the query returning a `Promise` which will be
   * resolved with either the doc(s) or rejected with the error.
   * Like `.then()`, but only takes a rejection handler.
   *
   * More about [Promise `catch()` in JavaScript](https://masteringjs.io/tutorials/fundamentals/catch).
   *
   * @param {Function} [reject]
   * @return {Promise}
   * @api public
   */
  
  Query.prototype.catch = function(reject) {
    return this.exec().then(null, reject);
  };
  
  /**
   * Add pre [middleware](/docs/middleware.html) to this query instance. Doesn't affect
   * other queries.
   *
   * ####Example:
   *
   *     const q1 = Question.find({ answer: 42 });
   *     q1.pre(function middleware() {
   *       console.log(this.getFilter());
   *     });
   *     await q1.exec(); // Prints "{ answer: 42 }"
   *
   *     // Doesn't print anything, because `middleware()` is only
   *     // registered on `q1`.
   *     await Question.find({ answer: 42 });
   *
   * @param {Function} fn
   * @return {Promise}
   * @api public
   */
  
  Query.prototype.pre = function(fn) {
    this._hooks.pre('exec', fn);
    return this;
  };
  
  /**
   * Add post [middleware](/docs/middleware.html) to this query instance. Doesn't affect
   * other queries.
   *
   * ####Example:
   *
   *     const q1 = Question.find({ answer: 42 });
   *     q1.post(function middleware() {
   *       console.log(this.getFilter());
   *     });
   *     await q1.exec(); // Prints "{ answer: 42 }"
   *
   *     // Doesn't print anything, because `middleware()` is only
   *     // registered on `q1`.
   *     await Question.find({ answer: 42 });
   *
   * @param {Function} fn
   * @return {Promise}
   * @api public
   */
  
  Query.prototype.post = function(fn) {
    this._hooks.post('exec', fn);
    return this;
  };
  
  /*!
   * Casts obj for an update command.
   *
   * @param {Object} obj
   * @return {Object} obj after casting its values
   * @api private
   */
  
  Query.prototype._castUpdate = function _castUpdate(obj, overwrite) {
    let strict;
    let schema = this.schema;
  
    const discriminatorKey = schema.options.discriminatorKey;
    const baseSchema = schema._baseSchema ? schema._baseSchema : schema;
    if (this._mongooseOptions.overwriteDiscriminatorKey &&
        obj[discriminatorKey] != null &&
        baseSchema.discriminators) {
      const _schema = baseSchema.discriminators[obj[discriminatorKey]];
      if (_schema != null) {
        schema = _schema;
      }
    }
  
    if ('strict' in this._mongooseOptions) {
      strict = this._mongooseOptions.strict;
    } else if (this.schema && this.schema.options) {
      strict = this.schema.options.strict;
    } else {
      strict = true;
    }
  
    let omitUndefined = false;
    if ('omitUndefined' in this._mongooseOptions) {
      omitUndefined = this._mongooseOptions.omitUndefined;
    }
  
    let useNestedStrict;
    if ('useNestedStrict' in this.options) {
      useNestedStrict = this.options.useNestedStrict;
    }
  
    let upsert;
    if ('upsert' in this.options) {
      upsert = this.options.upsert;
    }
  
    const filter = this._conditions;
    if (schema != null &&
        utils.hasUserDefinedProperty(filter, schema.options.discriminatorKey) &&
        typeof filter[schema.options.discriminatorKey] !== 'object' &&
        schema.discriminators != null) {
      const discriminatorValue = filter[schema.options.discriminatorKey];
      const byValue = getDiscriminatorByValue(this.model.discriminators, discriminatorValue);
      schema = schema.discriminators[discriminatorValue] ||
        (byValue && byValue.schema) ||
        schema;
    }
  
    return castUpdate(schema, obj, {
      overwrite: overwrite,
      strict: strict,
      omitUndefined,
      useNestedStrict: useNestedStrict,
      upsert: upsert,
      arrayFilters: this.options.arrayFilters
    }, this, this._conditions);
  };
  
  /*!
   * castQuery
   * @api private
   */
  
  function castQuery(query) {
    try {
      return query.cast(query.model);
    } catch (err) {
      return err;
    }
  }
  
  /*!
   * castDoc
   * @api private
   */
  
  function castDoc(query, overwrite) {
    try {
      return query._castUpdate(query._update, overwrite);
    } catch (err) {
      return err;
    }
  }
  
  /**
   * Specifies paths which should be populated with other documents.
   *
   * ####Example:
   *
   *     let book = await Book.findOne().populate('authors');
   *     book.title; // 'Node.js in Action'
   *     book.authors[0].name; // 'TJ Holowaychuk'
   *     book.authors[1].name; // 'Nathan Rajlich'
   *
   *     let books = await Book.find().populate({
   *       path: 'authors',
   *       // `match` and `sort` apply to the Author model,
   *       // not the Book model. These options do not affect
   *       // which documents are in `books`, just the order and
   *       // contents of each book document's `authors`.
   *       match: { name: new RegExp('.*h.*', 'i') },
   *       sort: { name: -1 }
   *     });
   *     books[0].title; // 'Node.js in Action'
   *     // Each book's `authors` are sorted by name, descending.
   *     books[0].authors[0].name; // 'TJ Holowaychuk'
   *     books[0].authors[1].name; // 'Marc Harter'
   *
   *     books[1].title; // 'Professional AngularJS'
   *     // Empty array, no authors' name has the letter 'h'
   *     books[1].authors; // []
   *
   * Paths are populated after the query executes and a response is received. A
   * separate query is then executed for each path specified for population. After
   * a response for each query has also been returned, the results are passed to
   * the callback.
   *
   * @param {Object|String} path either the path to populate or an object specifying all parameters
   * @param {Object|String} [select] Field selection for the population query
   * @param {Model} [model] The model you wish to use for population. If not specified, populate will look up the model by the name in the Schema's `ref` field.
   * @param {Object} [match] Conditions for the population query
   * @param {Object} [options] Options for the population query (sort, etc)
   * @param {String} [options.path=null] The path to populate.
   * @param {boolean} [options.retainNullValues=false] by default, Mongoose removes null and undefined values from populated arrays. Use this option to make `populate()` retain `null` and `undefined` array entries.
   * @param {boolean} [options.getters=false] if true, Mongoose will call any getters defined on the `localField`. By default, Mongoose gets the raw value of `localField`. For example, you would need to set this option to `true` if you wanted to [add a `lowercase` getter to your `localField`](/docs/schematypes.html#schematype-options).
   * @param {boolean} [options.clone=false] When you do `BlogPost.find().populate('author')`, blog posts with the same author will share 1 copy of an `author` doc. Enable this option to make Mongoose clone populated docs before assigning them.
   * @param {Object|Function} [options.match=null] Add an additional filter to the populate query. Can be a filter object containing [MongoDB query syntax](https://docs.mongodb.com/manual/tutorial/query-documents/), or a function that returns a filter object.
   * @param {Function} [options.transform=null] Function that Mongoose will call on every populated document that allows you to transform the populated document.
   * @param {Object} [options.options=null] Additional options like `limit` and `lean`.
   * @see population ./populate.html
   * @see Query#select #query_Query-select
   * @see Model.populate #model_Model.populate
   * @return {Query} this
   * @api public
   */
  
  Query.prototype.populate = function() {
    // Bail when given no truthy arguments
    if (!Array.from(arguments).some(Boolean)) {
      return this;
    }
  
    const res = utils.populate.apply(null, arguments);
  
    // Propagate readConcern and readPreference and lean from parent query,
    // unless one already specified
    if (this.options != null) {
      const readConcern = this.options.readConcern;
      const readPref = this.options.readPreference;
  
      for (const populateOptions of res) {
        if (readConcern != null && get(populateOptions, 'options.readConcern') == null) {
          populateOptions.options = populateOptions.options || {};
          populateOptions.options.readConcern = readConcern;
        }
        if (readPref != null && get(populateOptions, 'options.readPreference') == null) {
          populateOptions.options = populateOptions.options || {};
          populateOptions.options.readPreference = readPref;
        }
      }
    }
  
    const opts = this._mongooseOptions;
  
    if (opts.lean != null) {
      const lean = opts.lean;
      for (const populateOptions of res) {
        if (get(populateOptions, 'options.lean') == null) {
          populateOptions.options = populateOptions.options || {};
          populateOptions.options.lean = lean;
        }
      }
    }
  
    if (!utils.isObject(opts.populate)) {
      opts.populate = {};
    }
  
    const pop = opts.populate;
  
    for (const populateOptions of res) {
      const path = populateOptions.path;
      if (pop[path] && pop[path].populate && populateOptions.populate) {
        populateOptions.populate = pop[path].populate.concat(populateOptions.populate);
      }
  
      pop[populateOptions.path] = populateOptions;
    }
    return this;
  };
  
  /**
   * Gets a list of paths to be populated by this query
   *
   * ####Example:
   *      bookSchema.pre('findOne', function() {
   *        let keys = this.getPopulatedPaths(); // ['author']
   *      });
   *      ...
   *      Book.findOne({}).populate('author');
   *
   * ####Example:
   *      // Deep populate
   *      const q = L1.find().populate({
   *        path: 'level2',
   *        populate: { path: 'level3' }
   *      });
   *      q.getPopulatedPaths(); // ['level2', 'level2.level3']
   *
   * @return {Array} an array of strings representing populated paths
   * @api public
   */
  
  Query.prototype.getPopulatedPaths = function getPopulatedPaths() {
    const obj = this._mongooseOptions.populate || {};
    const ret = Object.keys(obj);
    for (const path of Object.keys(obj)) {
      const pop = obj[path];
      if (!Array.isArray(pop.populate)) {
        continue;
      }
      _getPopulatedPaths(ret, pop.populate, path + '.');
    }
    return ret;
  };
  
  /*!
   * ignore
   */
  
  function _getPopulatedPaths(list, arr, prefix) {
    for (const pop of arr) {
      list.push(prefix + pop.path);
      if (!Array.isArray(pop.populate)) {
        continue;
      }
      _getPopulatedPaths(list, pop.populate, prefix + pop.path + '.');
    }
  }
  
  /**
   * Casts this query to the schema of `model`
   *
   * ####Note
   *
   * If `obj` is present, it is cast instead of this query.
   *
   * @param {Model} [model] the model to cast to. If not set, defaults to `this.model`
   * @param {Object} [obj]
   * @return {Object}
   * @api public
   */
  
  Query.prototype.cast = function(model, obj) {
    obj || (obj = this._conditions);
  
    model = model || this.model;
    const discriminatorKey = model.schema.options.discriminatorKey;
    if (obj != null &&
        obj.hasOwnProperty(discriminatorKey)) {
      model = getDiscriminatorByValue(model.discriminators, obj[discriminatorKey]) || model;
    }
  
    try {
      return cast(model.schema, obj, {
        upsert: this.options && this.options.upsert,
        strict: (this.options && 'strict' in this.options) ?
          this.options.strict :
          get(model, 'schema.options.strict', null),
        strictQuery: (this.options && this.options.strictQuery) ||
          get(model, 'schema.options.strictQuery', null)
      }, this);
    } catch (err) {
      // CastError, assign model
      if (typeof err.setModel === 'function') {
        err.setModel(model);
      }
      throw err;
    }
  };
  
  /**
   * Casts selected field arguments for field selection with mongo 2.2
   *
   *     query.select({ ids: { $elemMatch: { $in: [hexString] }})
   *
   * @param {Object} fields
   * @see https://github.com/Automattic/mongoose/issues/1091
   * @see http://docs.mongodb.org/manual/reference/projection/elemMatch/
   * @api private
   */
  
  Query.prototype._castFields = function _castFields(fields) {
    let selected,
        elemMatchKeys,
        keys,
        key,
        out,
        i;
  
    if (fields) {
      keys = Object.keys(fields);
      elemMatchKeys = [];
      i = keys.length;
  
      // collect $elemMatch args
      while (i--) {
        key = keys[i];
        if (fields[key].$elemMatch) {
          selected || (selected = {});
          selected[key] = fields[key];
          elemMatchKeys.push(key);
        }
      }
    }
  
    if (selected) {
      // they passed $elemMatch, cast em
      try {
        out = this.cast(this.model, selected);
      } catch (err) {
        return err;
      }
  
      // apply the casted field args
      i = elemMatchKeys.length;
      while (i--) {
        key = elemMatchKeys[i];
        fields[key] = out[key];
      }
    }
  
    return fields;
  };
  
  /**
   * Applies schematype selected options to this query.
   * @api private
   */
  
  Query.prototype._applyPaths = function applyPaths() {
    this._fields = this._fields || {};
    helpers.applyPaths(this._fields, this.model.schema);
  
    let _selectPopulatedPaths = true;
  
    if ('selectPopulatedPaths' in this.model.base.options) {
      _selectPopulatedPaths = this.model.base.options.selectPopulatedPaths;
    }
    if ('selectPopulatedPaths' in this.model.schema.options) {
      _selectPopulatedPaths = this.model.schema.options.selectPopulatedPaths;
    }
  
    if (_selectPopulatedPaths) {
      selectPopulatedFields(this._fields, this._userProvidedFields, this._mongooseOptions.populate);
    }
  };
  
  /**
   * Returns a wrapper around a [mongodb driver cursor](http://mongodb.github.io/node-mongodb-native/2.1/api/Cursor.html).
   * A QueryCursor exposes a Streams3 interface, as well as a `.next()` function.
   *
   * The `.cursor()` function triggers pre find hooks, but **not** post find hooks.
   *
   * ####Example
   *
   *     // There are 2 ways to use a cursor. First, as a stream:
   *     Thing.
   *       find({ name: /^hello/ }).
   *       cursor().
   *       on('data', function(doc) { console.log(doc); }).
   *       on('end', function() { console.log('Done!'); });
   *
   *     // Or you can use `.next()` to manually get the next doc in the stream.
   *     // `.next()` returns a promise, so you can use promises or callbacks.
   *     const cursor = Thing.find({ name: /^hello/ }).cursor();
   *     cursor.next(function(error, doc) {
   *       console.log(doc);
   *     });
   *
   *     // Because `.next()` returns a promise, you can use co
   *     // to easily iterate through all documents without loading them
   *     // all into memory.
   *     co(function*() {
   *       const cursor = Thing.find({ name: /^hello/ }).cursor();
   *       for (let doc = yield cursor.next(); doc != null; doc = yield cursor.next()) {
   *         console.log(doc);
   *       }
   *     });
   *
   * ####Valid options
   *
   *   - `transform`: optional function which accepts a mongoose document. The return value of the function will be emitted on `data` and returned by `.next()`.
   *
   * @return {QueryCursor}
   * @param {Object} [options]
   * @see QueryCursor
   * @api public
   */
  
  Query.prototype.cursor = function cursor(opts) {
    this._applyPaths();
    this._fields = this._castFields(this._fields);
    this.setOptions({ projection: this._fieldsForExec() });
    if (opts) {
      this.setOptions(opts);
    }
  
    const options = Object.assign({}, this._optionsForExec(), {
      projection: this.projection()
    });
    try {
      this.cast(this.model);
    } catch (err) {
      return (new QueryCursor(this, options))._markError(err);
    }
  
    return new QueryCursor(this, options);
  };
  
  // the rest of these are basically to support older Mongoose syntax with mquery
  
  /**
   * _DEPRECATED_ Alias of `maxScan`
   *
   * @deprecated
   * @see maxScan #query_Query-maxScan
   * @method maxscan
   * @memberOf Query
   * @instance
   */
  
  Query.prototype.maxscan = Query.base.maxScan;
  
  /**
   * Sets the tailable option (for use with capped collections).
   *
   * ####Example
   *
   *     query.tailable() // true
   *     query.tailable(true)
   *     query.tailable(false)
   *
   * ####Note
   *
   * Cannot be used with `distinct()`
   *
   * @param {Boolean} bool defaults to true
   * @param {Object} [opts] options to set
   * @param {Number} [opts.numberOfRetries] if cursor is exhausted, retry this many times before giving up
   * @param {Number} [opts.tailableRetryInterval] if cursor is exhausted, wait this many milliseconds before retrying
   * @see tailable http://docs.mongodb.org/manual/tutorial/create-tailable-cursor/
   * @api public
   */
  
  Query.prototype.tailable = function(val, opts) {
    // we need to support the tailable({ awaitdata : true }) as well as the
    // tailable(true, {awaitdata :true}) syntax that mquery does not support
    if (val && val.constructor.name === 'Object') {
      opts = val;
      val = true;
    }
  
    if (val === undefined) {
      val = true;
    }
  
    if (opts && typeof opts === 'object') {
      for (const key of Object.keys(opts)) {
        if (key === 'awaitdata') {
          // For backwards compatibility
          this.options[key] = !!opts[key];
        } else {
          this.options[key] = opts[key];
        }
      }
    }
  
    return Query.base.tailable.call(this, val);
  };
  
  /**
   * Declares an intersects query for `geometry()`.
   *
   * ####Example
   *
   *     query.where('path').intersects().geometry({
   *         type: 'LineString'
   *       , coordinates: [[180.0, 11.0], [180, 9.0]]
   *     })
   *
   *     query.where('path').intersects({
   *         type: 'LineString'
   *       , coordinates: [[180.0, 11.0], [180, 9.0]]
   *     })
   *
   * ####NOTE:
   *
   * **MUST** be used after `where()`.
   *
   * ####NOTE:
   *
   * In Mongoose 3.7, `intersects` changed from a getter to a function. If you need the old syntax, use [this](https://github.com/ebensing/mongoose-within).
   *
   * @method intersects
   * @memberOf Query
   * @instance
   * @param {Object} [arg]
   * @return {Query} this
   * @see $geometry http://docs.mongodb.org/manual/reference/operator/geometry/
   * @see geoIntersects http://docs.mongodb.org/manual/reference/operator/geoIntersects/
   * @api public
   */
  
  /**
   * Specifies a `$geometry` condition
   *
   * ####Example
   *
   *     const polyA = [[[ 10, 20 ], [ 10, 40 ], [ 30, 40 ], [ 30, 20 ]]]
   *     query.where('loc').within().geometry({ type: 'Polygon', coordinates: polyA })
   *
   *     // or
   *     const polyB = [[ 0, 0 ], [ 1, 1 ]]
   *     query.where('loc').within().geometry({ type: 'LineString', coordinates: polyB })
   *
   *     // or
   *     const polyC = [ 0, 0 ]
   *     query.where('loc').within().geometry({ type: 'Point', coordinates: polyC })
   *
   *     // or
   *     query.where('loc').intersects().geometry({ type: 'Point', coordinates: polyC })
   *
   * The argument is assigned to the most recent path passed to `where()`.
   *
   * ####NOTE:
   *
   * `geometry()` **must** come after either `intersects()` or `within()`.
   *
   * The `object` argument must contain `type` and `coordinates` properties.
   * - type {String}
   * - coordinates {Array}
   *
   * @method geometry
   * @memberOf Query
   * @instance
   * @param {Object} object Must contain a `type` property which is a String and a `coordinates` property which is an Array. See the examples.
   * @return {Query} this
   * @see $geometry http://docs.mongodb.org/manual/reference/operator/geometry/
   * @see http://docs.mongodb.org/manual/release-notes/2.4/#new-geospatial-indexes-with-geojson-and-improved-spherical-geometry
   * @see http://www.mongodb.org/display/DOCS/Geospatial+Indexing
   * @api public
   */
  
  /**
   * Specifies a `$near` or `$nearSphere` condition
   *
   * These operators return documents sorted by distance.
   *
   * ####Example
   *
   *     query.where('loc').near({ center: [10, 10] });
   *     query.where('loc').near({ center: [10, 10], maxDistance: 5 });
   *     query.where('loc').near({ center: [10, 10], maxDistance: 5, spherical: true });
   *     query.near('loc', { center: [10, 10], maxDistance: 5 });
   *
   * @method near
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Object} val
   * @return {Query} this
   * @see $near http://docs.mongodb.org/manual/reference/operator/near/
   * @see $nearSphere http://docs.mongodb.org/manual/reference/operator/nearSphere/
   * @see $maxDistance http://docs.mongodb.org/manual/reference/operator/maxDistance/
   * @see http://www.mongodb.org/display/DOCS/Geospatial+Indexing
   * @api public
   */
  
  /*!
   * Overwriting mquery is needed to support a couple different near() forms found in older
   * versions of mongoose
   * near([1,1])
   * near(1,1)
   * near(field, [1,2])
   * near(field, 1, 2)
   * In addition to all of the normal forms supported by mquery
   */
  
  Query.prototype.near = function() {
    const params = [];
    const sphere = this._mongooseOptions.nearSphere;
  
    // TODO refactor
  
    if (arguments.length === 1) {
      if (Array.isArray(arguments[0])) {
        params.push({ center: arguments[0], spherical: sphere });
      } else if (typeof arguments[0] === 'string') {
        // just passing a path
        params.push(arguments[0]);
      } else if (utils.isObject(arguments[0])) {
        if (typeof arguments[0].spherical !== 'boolean') {
          arguments[0].spherical = sphere;
        }
        params.push(arguments[0]);
      } else {
        throw new TypeError('invalid argument');
      }
    } else if (arguments.length === 2) {
      if (typeof arguments[0] === 'number' && typeof arguments[1] === 'number') {
        params.push({ center: [arguments[0], arguments[1]], spherical: sphere });
      } else if (typeof arguments[0] === 'string' && Array.isArray(arguments[1])) {
        params.push(arguments[0]);
        params.push({ center: arguments[1], spherical: sphere });
      } else if (typeof arguments[0] === 'string' && utils.isObject(arguments[1])) {
        params.push(arguments[0]);
        if (typeof arguments[1].spherical !== 'boolean') {
          arguments[1].spherical = sphere;
        }
        params.push(arguments[1]);
      } else {
        throw new TypeError('invalid argument');
      }
    } else if (arguments.length === 3) {
      if (typeof arguments[0] === 'string' && typeof arguments[1] === 'number'
          && typeof arguments[2] === 'number') {
        params.push(arguments[0]);
        params.push({ center: [arguments[1], arguments[2]], spherical: sphere });
      } else {
        throw new TypeError('invalid argument');
      }
    } else {
      throw new TypeError('invalid argument');
    }
  
    return Query.base.near.apply(this, params);
  };
  
  /**
   * _DEPRECATED_ Specifies a `$nearSphere` condition
   *
   * ####Example
   *
   *     query.where('loc').nearSphere({ center: [10, 10], maxDistance: 5 });
   *
   * **Deprecated.** Use `query.near()` instead with the `spherical` option set to `true`.
   *
   * ####Example
   *
   *     query.where('loc').near({ center: [10, 10], spherical: true });
   *
   * @deprecated
   * @see near() #query_Query-near
   * @see $near http://docs.mongodb.org/manual/reference/operator/near/
   * @see $nearSphere http://docs.mongodb.org/manual/reference/operator/nearSphere/
   * @see $maxDistance http://docs.mongodb.org/manual/reference/operator/maxDistance/
   */
  
  Query.prototype.nearSphere = function() {
    this._mongooseOptions.nearSphere = true;
    this.near.apply(this, arguments);
    return this;
  };
  
  /**
   * Returns an asyncIterator for use with [`for/await/of` loops](https://thecodebarbarian.com/getting-started-with-async-iterators-in-node-js)
   * This function *only* works for `find()` queries.
   * You do not need to call this function explicitly, the JavaScript runtime
   * will call it for you.
   *
   * ####Example
   *
   *     for await (const doc of Model.aggregate([{ $sort: { name: 1 } }])) {
   *       console.log(doc.name);
   *     }
   *
   * Node.js 10.x supports async iterators natively without any flags. You can
   * enable async iterators in Node.js 8.x using the [`--harmony_async_iteration` flag](https://github.com/tc39/proposal-async-iteration/issues/117#issuecomment-346695187).
   *
   * **Note:** This function is not if `Symbol.asyncIterator` is undefined. If
   * `Symbol.asyncIterator` is undefined, that means your Node.js version does not
   * support async iterators.
   *
   * @method Symbol.asyncIterator
   * @memberOf Query
   * @instance
   * @api public
   */
  
  if (Symbol.asyncIterator != null) {
    Query.prototype[Symbol.asyncIterator] = function() {
      return this.cursor().transformNull()._transformForAsyncIterator();
    };
  }
  
  /**
   * Specifies a `$polygon` condition
   *
   * ####Example
   *
   *     query.where('loc').within().polygon([10,20], [13, 25], [7,15])
   *     query.polygon('loc', [10,20], [13, 25], [7,15])
   *
   * @method polygon
   * @memberOf Query
   * @instance
   * @param {String|Array} [path]
   * @param {Array|Object} [coordinatePairs...]
   * @return {Query} this
   * @see $polygon http://docs.mongodb.org/manual/reference/operator/polygon/
   * @see http://www.mongodb.org/display/DOCS/Geospatial+Indexing
   * @api public
   */
  
  /**
   * Specifies a `$box` condition
   *
   * ####Example
   *
   *     const lowerLeft = [40.73083, -73.99756]
   *     const upperRight= [40.741404,  -73.988135]
   *
   *     query.where('loc').within().box(lowerLeft, upperRight)
   *     query.box({ ll : lowerLeft, ur : upperRight })
   *
   * @method box
   * @memberOf Query
   * @instance
   * @see $box http://docs.mongodb.org/manual/reference/operator/box/
   * @see within() Query#within #query_Query-within
   * @see http://www.mongodb.org/display/DOCS/Geospatial+Indexing
   * @param {Object} val
   * @param [Array] Upper Right Coords
   * @return {Query} this
   * @api public
   */
  
  /*!
   * this is needed to support the mongoose syntax of:
   * box(field, { ll : [x,y], ur : [x2,y2] })
   * box({ ll : [x,y], ur : [x2,y2] })
   */
  
  Query.prototype.box = function(ll, ur) {
    if (!Array.isArray(ll) && utils.isObject(ll)) {
      ur = ll.ur;
      ll = ll.ll;
    }
    return Query.base.box.call(this, ll, ur);
  };
  
  /**
   * Specifies a `$center` or `$centerSphere` condition.
   *
   * ####Example
   *
   *     const area = { center: [50, 50], radius: 10, unique: true }
   *     query.where('loc').within().circle(area)
   *     // alternatively
   *     query.circle('loc', area);
   *
   *     // spherical calculations
   *     const area = { center: [50, 50], radius: 10, unique: true, spherical: true }
   *     query.where('loc').within().circle(area)
   *     // alternatively
   *     query.circle('loc', area);
   *
   * @method circle
   * @memberOf Query
   * @instance
   * @param {String} [path]
   * @param {Object} area
   * @return {Query} this
   * @see $center http://docs.mongodb.org/manual/reference/operator/center/
   * @see $centerSphere http://docs.mongodb.org/manual/reference/operator/centerSphere/
   * @see $geoWithin http://docs.mongodb.org/manual/reference/operator/geoWithin/
   * @see http://www.mongodb.org/display/DOCS/Geospatial+Indexing
   * @api public
   */
  
  /**
   * _DEPRECATED_ Alias for [circle](#query_Query-circle)
   *
   * **Deprecated.** Use [circle](#query_Query-circle) instead.
   *
   * @deprecated
   * @method center
   * @memberOf Query
   * @instance
   * @api public
   */
  
  Query.prototype.center = Query.base.circle;
  
  /**
   * _DEPRECATED_ Specifies a `$centerSphere` condition
   *
   * **Deprecated.** Use [circle](#query_Query-circle) instead.
   *
   * ####Example
   *
   *     const area = { center: [50, 50], radius: 10 };
   *     query.where('loc').within().centerSphere(area);
   *
   * @deprecated
   * @param {String} [path]
   * @param {Object} val
   * @return {Query} this
   * @see http://www.mongodb.org/display/DOCS/Geospatial+Indexing
   * @see $centerSphere http://docs.mongodb.org/manual/reference/operator/centerSphere/
   * @api public
   */
  
  Query.prototype.centerSphere = function() {
    if (arguments[0] && arguments[0].constructor.name === 'Object') {
      arguments[0].spherical = true;
    }
  
    if (arguments[1] && arguments[1].constructor.name === 'Object') {
      arguments[1].spherical = true;
    }
  
    Query.base.circle.apply(this, arguments);
  };
  
  /**
   * Determines if field selection has been made.
   *
   * @method selected
   * @memberOf Query
   * @instance
   * @return {Boolean}
   * @api public
   */
  
  /**
   * Determines if inclusive field selection has been made.
   *
   *     query.selectedInclusively() // false
   *     query.select('name')
   *     query.selectedInclusively() // true
   *
   * @method selectedInclusively
   * @memberOf Query
   * @instance
   * @return {Boolean}
   * @api public
   */
  
  Query.prototype.selectedInclusively = function selectedInclusively() {
    return isInclusive(this._fields);
  };
  
  /**
   * Determines if exclusive field selection has been made.
   *
   *     query.selectedExclusively() // false
   *     query.select('-name')
   *     query.selectedExclusively() // true
   *     query.selectedInclusively() // false
   *
   * @method selectedExclusively
   * @memberOf Query
   * @instance
   * @return {Boolean}
   * @api public
   */
  
  Query.prototype.selectedExclusively = function selectedExclusively() {
    return isExclusive(this._fields);
  };
  
  /*!
   * Export
   */
  
  module.exports = Query;
  