text: 'use strict';
  
  /*!
   * Module dependencies
   */
  
  const AggregationCursor = require('./cursor/AggregationCursor');
  const Query = require('./query');
  const applyGlobalMaxTimeMS = require('./helpers/query/applyGlobalMaxTimeMS');
  const getConstructorName = require('./helpers/getConstructorName');
  const promiseOrCallback = require('./helpers/promiseOrCallback');
  const stringifyFunctionOperators = require('./helpers/aggregate/stringifyFunctionOperators');
  const util = require('util');
  const utils = require('./utils');
  const read = Query.prototype.read;
  const readConcern = Query.prototype.readConcern;
  
  /**
   * Aggregate constructor used for building aggregation pipelines. Do not
   * instantiate this class directly, use [Model.aggregate()](/docs/api.html#model_Model.aggregate) instead.
   *
   * ####Example:
   *
   *     const aggregate = Model.aggregate([
   *       { $project: { a: 1, b: 1 } },
   *       { $skip: 5 }
   *     ]);
   *
   *     Model.
   *       aggregate([{ $match: { age: { $gte: 21 }}}]).
   *       unwind('tags').
   *       exec(callback);
   *
   * ####Note:
   *
   * - The documents returned are plain javascript objects, not mongoose documents (since any shape of document can be returned).
   * - Mongoose does **not** cast pipeline stages. The below will **not** work unless `_id` is a string in the database
   *
   * ```javascript
   *   new Aggregate([{ $match: { _id: '00000000000000000000000a' } }]);
   *   // Do this instead to cast to an ObjectId
   *   new Aggregate([{ $match: { _id: mongoose.Types.ObjectId('00000000000000000000000a') } }]);
   * ```
   *
   * @see MongoDB http://docs.mongodb.org/manual/applications/aggregation/
   * @see driver http://mongodb.github.com/node-mongodb-native/api-generated/collection.html#aggregate
   * @param {Array} [pipeline] aggregation pipeline as an array of objects
   * @param {Model} [model] the model to use with this aggregate.
   * @api public
   */
  
  function Aggregate(pipeline, model) {
    this._pipeline = [];
    this._model = model;
    this.options = {};
  
    if (arguments.length === 1 && util.isArray(pipeline)) {
      this.append.apply(this, pipeline);
    }
  }
  
  /**
   * Contains options passed down to the [aggregate command](https://docs.mongodb.com/manual/reference/command/aggregate/).
   * Supported options are:
   *
   * - `readPreference`
   * - [`cursor`](./api.html#aggregate_Aggregate-cursor)
   * - [`explain`](./api.html#aggregate_Aggregate-explain)
   * - [`allowDiskUse`](./api.html#aggregate_Aggregate-allowDiskUse)
   * - `maxTimeMS`
   * - `bypassDocumentValidation`
   * - `raw`
   * - `promoteLongs`
   * - `promoteValues`
   * - `promoteBuffers`
   * - [`collation`](./api.html#aggregate_Aggregate-collation)
   * - `comment`
   * - [`session`](./api.html#aggregate_Aggregate-session)
   *
   * @property options
   * @memberOf Aggregate
   * @api public
   */
  
  Aggregate.prototype.options;
  
  /**
   * Get/set the model that this aggregation will execute on.
   *
   * ####Example:
   *     const aggregate = MyModel.aggregate([{ $match: { answer: 42 } }]);
   *     aggregate.model() === MyModel; // true
   *
   *     // Change the model. There's rarely any reason to do this.
   *     aggregate.model(SomeOtherModel);
   *     aggregate.model() === SomeOtherModel; // true
   *
   * @param {Model} [model] set the model associated with this aggregate.
   * @return {Model}
   * @api public
   */
  
  Aggregate.prototype.model = function(model) {
    if (arguments.length === 0) {
      return this._model;
    }
  
    this._model = model;
    if (model.schema != null) {
      if (this.options.readPreference == null &&
          model.schema.options.read != null) {
        this.options.readPreference = model.schema.options.read;
      }
      if (this.options.collation == null &&
          model.schema.options.collation != null) {
        this.options.collation = model.schema.options.collation;
      }
    }
  
    return model;
  };
  
  /**
   * Appends new operators to this aggregate pipeline
   *
   * ####Examples:
   *
   *     aggregate.append({ $project: { field: 1 }}, { $limit: 2 });
   *
   *     // or pass an array
   *     const pipeline = [{ $match: { daw: 'Logic Audio X' }} ];
   *     aggregate.append(pipeline);
   *
   * @param {Object} ops operator(s) to append
   * @return {Aggregate}
   * @api public
   */
  
  Aggregate.prototype.append = function() {
    const args = (arguments.length === 1 && util.isArray(arguments[0]))
      ? arguments[0]
      : utils.args(arguments);
  
    if (!args.every(isOperator)) {
      throw new Error('Arguments must be aggregate pipeline operators');
    }
  
    this._pipeline = this._pipeline.concat(args);
  
    return this;
  };
  
  /**
   * Appends a new $addFields operator to this aggregate pipeline.
   * Requires MongoDB v3.4+ to work
   *
   * ####Examples:
    *
   *     // adding new fields based on existing fields
   *     aggregate.addFields({
   *         newField: '$b.nested'
   *       , plusTen: { $add: ['$val', 10]}
   *       , sub: {
   *            name: '$a'
   *         }
   *     })
   *
   *     // etc
   *     aggregate.addFields({ salary_k: { $divide: [ "$salary", 1000 ] } });
   *
   * @param {Object} arg field specification
   * @see $addFields https://docs.mongodb.com/manual/reference/operator/aggregation/addFields/
   * @return {Aggregate}
   * @api public
   */
  Aggregate.prototype.addFields = function(arg) {
    const fields = {};
    if (typeof arg === 'object' && !util.isArray(arg)) {
      Object.keys(arg).forEach(function(field) {
        fields[field] = arg[field];
      });
    } else {
      throw new Error('Invalid addFields() argument. Must be an object');
    }
    return this.append({ $addFields: fields });
  };
  
  /**
   * Appends a new $project operator to this aggregate pipeline.
   *
   * Mongoose query [selection syntax](#query_Query-select) is also supported.
   *
   * ####Examples:
   *
   *     // include a, include b, exclude _id
   *     aggregate.project("a b -_id");
   *
   *     // or you may use object notation, useful when
   *     // you have keys already prefixed with a "-"
   *     aggregate.project({a: 1, b: 1, _id: 0});
   *
   *     // reshaping documents
   *     aggregate.project({
   *         newField: '$b.nested'
   *       , plusTen: { $add: ['$val', 10]}
   *       , sub: {
   *            name: '$a'
   *         }
   *     })
   *
   *     // etc
   *     aggregate.project({ salary_k: { $divide: [ "$salary", 1000 ] } });
   *
   * @param {Object|String} arg field specification
   * @see projection http://docs.mongodb.org/manual/reference/aggregation/project/
   * @return {Aggregate}
   * @api public
   */
  
  Aggregate.prototype.project = function(arg) {
    const fields = {};
  
    if (typeof arg === 'object' && !util.isArray(arg)) {
      Object.keys(arg).forEach(function(field) {
        fields[field] = arg[field];
      });
    } else if (arguments.length === 1 && typeof arg === 'string') {
      arg.split(/\s+/).forEach(function(field) {
        if (!field) {
          return;
        }
        const include = field[0] === '-' ? 0 : 1;
        if (include === 0) {
          field = field.substring(1);
        }
        fields[field] = include;
      });
    } else {
      throw new Error('Invalid project() argument. Must be string or object');
    }
  
    return this.append({ $project: fields });
  };
  
  /**
   * Appends a new custom $group operator to this aggregate pipeline.
   *
   * ####Examples:
   *
   *     aggregate.group({ _id: "$department" });
   *
   * @see $group http://docs.mongodb.org/manual/reference/aggregation/group/
   * @method group
   * @memberOf Aggregate
   * @instance
   * @param {Object} arg $group operator contents
   * @return {Aggregate}
   * @api public
   */
  
  /**
   * Appends a new custom $match operator to this aggregate pipeline.
   *
   * ####Examples:
   *
   *     aggregate.match({ department: { $in: [ "sales", "engineering" ] } });
   *
   * @see $match http://docs.mongodb.org/manual/reference/aggregation/match/
   * @method match
   * @memberOf Aggregate
   * @instance
   * @param {Object} arg $match operator contents
   * @return {Aggregate}
   * @api public
   */
  
  /**
   * Appends a new $skip operator to this aggregate pipeline.
   *
   * ####Examples:
   *
   *     aggregate.skip(10);
   *
   * @see $skip http://docs.mongodb.org/manual/reference/aggregation/skip/
   * @method skip
   * @memberOf Aggregate
   * @instance
   * @param {Number} num number of records to skip before next stage
   * @return {Aggregate}
   * @api public
   */
  
  /**
   * Appends a new $limit operator to this aggregate pipeline.
   *
   * ####Examples:
   *
   *     aggregate.limit(10);
   *
   * @see $limit http://docs.mongodb.org/manual/reference/aggregation/limit/
   * @method limit
   * @memberOf Aggregate
   * @instance
   * @param {Number} num maximum number of records to pass to the next stage
   * @return {Aggregate}
   * @api public
   */
  
  /**
   * Appends a new $geoNear operator to this aggregate pipeline.
   *
   * ####NOTE:
   *
   * **MUST** be used as the first operator in the pipeline.
   *
   * ####Examples:
   *
   *     aggregate.near({
   *       near: [40.724, -73.997],
   *       distanceField: "dist.calculated", // required
   *       maxDistance: 0.008,
   *       query: { type: "public" },
   *       includeLocs: "dist.location",
   *       uniqueDocs: true,
   *       num: 5
   *     });
   *
   * @see $geoNear http://docs.mongodb.org/manual/reference/aggregation/geoNear/
   * @method near
   * @memberOf Aggregate
   * @instance
   * @param {Object} arg
   * @return {Aggregate}
   * @api public
   */
  
  Aggregate.prototype.near = function(arg) {
    const op = {};
    op.$geoNear = arg;
    return this.append(op);
  };
  
  /*!
   * define methods
   */
  
  'group match skip limit out'.split(' ').forEach(function($operator) {
    Aggregate.prototype[$operator] = function(arg) {
      const op = {};
      op['$' + $operator] = arg;
      return this.append(op);
    };
  });
  
  /**
   * Appends new custom $unwind operator(s) to this aggregate pipeline.
   *
   * Note that the `$unwind` operator requires the path name to start with '$'.
   * Mongoose will prepend '$' if the specified field doesn't start '$'.
   *
   * ####Examples:
   *
   *     aggregate.unwind("tags");
   *     aggregate.unwind("a", "b", "c");
   *     aggregate.unwind({ path: '$tags', preserveNullAndEmptyArrays: true });
   *
   * @see $unwind http://docs.mongodb.org/manual/reference/aggregation/unwind/
   * @param {String|Object} fields the field(s) to unwind, either as field names or as [objects with options](https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/#document-operand-with-options). If passing a string, prefixing the field name with '$' is optional. If passing an object, `path` must start with '$'.
   * @return {Aggregate}
   * @api public
   */
  
  Aggregate.prototype.unwind = function() {
    const args = utils.args(arguments);
  
    const res = [];
    for (const arg of args) {
      if (arg && typeof arg === 'object') {
        res.push({ $unwind: arg });
      } else if (typeof arg === 'string') {
        res.push({
          $unwind: (arg && arg.startsWith('$')) ? arg : '$' + arg
        });
      } else {
        throw new Error('Invalid arg "' + arg + '" to unwind(), ' +
          'must be string or object');
      }
    }
  
    return this.append.apply(this, res);
  };
  
  /**
   * Appends a new $replaceRoot operator to this aggregate pipeline.
   *
   * Note that the `$replaceRoot` operator requires field strings to start with '$'.
   * If you are passing in a string Mongoose will prepend '$' if the specified field doesn't start '$'.
   * If you are passing in an object the strings in your expression will not be altered.
   *
   * ####Examples:
   *
   *     aggregate.replaceRoot("user");
   *
   *     aggregate.replaceRoot({ x: { $concat: ['$this', '$that'] } });
   *
   * @see $replaceRoot https://docs.mongodb.org/manual/reference/operator/aggregation/replaceRoot
   * @param {String|Object} the field or document which will become the new root document
   * @return {Aggregate}
   * @api public
   */
  
  Aggregate.prototype.replaceRoot = function(newRoot) {
    let ret;
  
    if (typeof newRoot === 'string') {
      ret = newRoot.startsWith('$') ? newRoot : '$' + newRoot;
    } else {
      ret = newRoot;
    }
  
    return this.append({
      $replaceRoot: {
        newRoot: ret
      }
    });
  };
  
  /**
   * Appends a new $count operator to this aggregate pipeline.
   *
   * ####Examples:
   *
   *     aggregate.count("userCount");
   *
   * @see $count https://docs.mongodb.org/manual/reference/operator/aggregation/count
   * @param {String} the name of the count field
   * @return {Aggregate}
   * @api public
   */
  
  Aggregate.prototype.count = function(countName) {
    return this.append({ $count: countName });
  };
  
  /**
   * Appends a new $sortByCount operator to this aggregate pipeline. Accepts either a string field name
   * or a pipeline object.
   *
   * Note that the `$sortByCount` operator requires the new root to start with '$'.
   * Mongoose will prepend '$' if the specified field name doesn't start with '$'.
   *
   * ####Examples:
   *
   *     aggregate.sortByCount('users');
   *     aggregate.sortByCount({ $mergeObjects: [ "$employee", "$business" ] })
   *
   * @see $sortByCount https://docs.mongodb.com/manual/reference/operator/aggregation/sortByCount/
   * @param {Object|String} arg
   * @return {Aggregate} this
   * @api public
   */
  
  Aggregate.prototype.sortByCount = function(arg) {
    if (arg && typeof arg === 'object') {
      return this.append({ $sortByCount: arg });
    } else if (typeof arg === 'string') {
      return this.append({
        $sortByCount: (arg && arg.startsWith('$')) ? arg : '$' + arg
      });
    } else {
      throw new TypeError('Invalid arg "' + arg + '" to sortByCount(), ' +
        'must be string or object');
    }
  };
  
  /**
   * Appends new custom $lookup operator to this aggregate pipeline.
   *
   * ####Examples:
   *
   *     aggregate.lookup({ from: 'users', localField: 'userId', foreignField: '_id', as: 'users' });
   *
   * @see $lookup https://docs.mongodb.org/manual/reference/operator/aggregation/lookup/#pipe._S_lookup
   * @param {Object} options to $lookup as described in the above link
   * @return {Aggregate}* @api public
   */
  
  Aggregate.prototype.lookup = function(options) {
    return this.append({ $lookup: options });
  };
  
  /**
   * Appends new custom $graphLookup operator(s) to this aggregate pipeline, performing a recursive search on a collection.
   *
   * Note that graphLookup can only consume at most 100MB of memory, and does not allow disk use even if `{ allowDiskUse: true }` is specified.
   *
   * #### Examples:
   *      // Suppose we have a collection of courses, where a document might look like `{ _id: 0, name: 'Calculus', prerequisite: 'Trigonometry'}` and `{ _id: 0, name: 'Trigonometry', prerequisite: 'Algebra' }`
   *      aggregate.graphLookup({ from: 'courses', startWith: '$prerequisite', connectFromField: 'prerequisite', connectToField: 'name', as: 'prerequisites', maxDepth: 3 }) // this will recursively search the 'courses' collection up to 3 prerequisites
   *
   * @see $graphLookup https://docs.mongodb.com/manual/reference/operator/aggregation/graphLookup/#pipe._S_graphLookup
   * @param {Object} options to $graphLookup as described in the above link
   * @return {Aggregate}
   * @api public
   */
  
  Aggregate.prototype.graphLookup = function(options) {
    const cloneOptions = {};
    if (options) {
      if (!utils.isObject(options)) {
        throw new TypeError('Invalid graphLookup() argument. Must be an object.');
      }
  
      utils.mergeClone(cloneOptions, options);
      const startWith = cloneOptions.startWith;
  
      if (startWith && typeof startWith === 'string') {
        cloneOptions.startWith = cloneOptions.startWith.startsWith('$') ?
          cloneOptions.startWith :
          '$' + cloneOptions.startWith;
      }
  
    }
    return this.append({ $graphLookup: cloneOptions });
  };
  
  /**
   * Appends new custom $sample operator to this aggregate pipeline.
   *
   * ####Examples:
   *
   *     aggregate.sample(3); // Add a pipeline that picks 3 random documents
   *
   * @see $sample https://docs.mongodb.org/manual/reference/operator/aggregation/sample/#pipe._S_sample
   * @param {Number} size number of random documents to pick
   * @return {Aggregate}
   * @api public
   */
  
  Aggregate.prototype.sample = function(size) {
    return this.append({ $sample: { size: size } });
  };
  
  /**
   * Appends a new $sort operator to this aggregate pipeline.
   *
   * 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.
   *
   * ####Examples:
   *
   *     // these are equivalent
   *     aggregate.sort({ field: 'asc', test: -1 });
   *     aggregate.sort('field -test');
   *
   * @see $sort http://docs.mongodb.org/manual/reference/aggregation/sort/
   * @param {Object|String} arg
   * @return {Aggregate} this
   * @api public
   */
  
  Aggregate.prototype.sort = function(arg) {
    // TODO refactor to reuse the query builder logic
  
    const sort = {};
  
    if (getConstructorName(arg) === 'Object') {
      const desc = ['desc', 'descending', -1];
      Object.keys(arg).forEach(function(field) {
        // If sorting by text score, skip coercing into 1/-1
        if (arg[field] instanceof Object && arg[field].$meta) {
          sort[field] = arg[field];
          return;
        }
        sort[field] = desc.indexOf(arg[field]) === -1 ? 1 : -1;
      });
    } else if (arguments.length === 1 && typeof arg === 'string') {
      arg.split(/\s+/).forEach(function(field) {
        if (!field) {
          return;
        }
        const ascend = field[0] === '-' ? -1 : 1;
        if (ascend === -1) {
          field = field.substring(1);
        }
        sort[field] = ascend;
      });
    } else {
      throw new TypeError('Invalid sort() argument. Must be a string or object.');
    }
  
    return this.append({ $sort: sort });
  };
  
  /**
   * Sets the readPreference option for the aggregation query.
   *
   * ####Example:
   *
   *     Model.aggregate(..).read('primaryPreferred').exec(callback)
   *
   * @param {String} pref one of the listed preference options or their aliases
   * @param {Array} [tags] optional tags for this query
   * @return {Aggregate} this
   * @api public
   * @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
   */
  
  Aggregate.prototype.read = function(pref, tags) {
    if (!this.options) {
      this.options = {};
    }
    read.call(this, pref, tags);
    return this;
  };
  
  /**
   * Sets the readConcern level for the aggregation query.
   *
   * ####Example:
   *
   *     Model.aggregate(..).readConcern('majority').exec(callback)
   *
   * @param {String} level one of the listed read concern level or their aliases
   * @see mongodb https://docs.mongodb.com/manual/reference/read-concern/
   * @return {Aggregate} this
   * @api public
   */
  
  Aggregate.prototype.readConcern = function(level) {
    if (!this.options) {
      this.options = {};
    }
    readConcern.call(this, level);
    return this;
  };
  
  /**
   * Appends a new $redact operator to this aggregate pipeline.
   *
   * If 3 arguments are supplied, Mongoose will wrap them with if-then-else of $cond operator respectively
   * If `thenExpr` or `elseExpr` is string, make sure it starts with $$, like `$$DESCEND`, `$$PRUNE` or `$$KEEP`.
   *
   * ####Example:
   *
   *     Model.aggregate(...)
   *      .redact({
   *        $cond: {
   *          if: { $eq: [ '$level', 5 ] },
   *          then: '$$PRUNE',
   *          else: '$$DESCEND'
   *        }
   *      })
   *      .exec();
   *
   *     // $redact often comes with $cond operator, you can also use the following syntax provided by mongoose
   *     Model.aggregate(...)
   *      .redact({ $eq: [ '$level', 5 ] }, '$$PRUNE', '$$DESCEND')
   *      .exec();
   *
   * @param {Object} expression redact options or conditional expression
   * @param {String|Object} [thenExpr] true case for the condition
   * @param {String|Object} [elseExpr] false case for the condition
   * @return {Aggregate} this
   * @see $redact https://docs.mongodb.com/manual/reference/operator/aggregation/redact/
   * @api public
   */
  
  Aggregate.prototype.redact = function(expression, thenExpr, elseExpr) {
    if (arguments.length === 3) {
      if ((typeof thenExpr === 'string' && !thenExpr.startsWith('$$')) ||
          (typeof elseExpr === 'string' && !elseExpr.startsWith('$$'))) {
        throw new Error('If thenExpr or elseExpr is string, it must start with $$. e.g. $$DESCEND, $$PRUNE, $$KEEP');
      }
  
      expression = {
        $cond: {
          if: expression,
          then: thenExpr,
          else: elseExpr
        }
      };
    } else if (arguments.length !== 1) {
      throw new TypeError('Invalid arguments');
    }
  
    return this.append({ $redact: expression });
  };
  
  /**
   * Execute the aggregation with explain
   *
   * ####Example:
   *
   *     Model.aggregate(..).explain(callback)
   *
   * @param {Function} callback
   * @return {Promise}
   */
  
  Aggregate.prototype.explain = function(callback) {
    const model = this._model;
  
    return promiseOrCallback(callback, cb => {
      if (!this._pipeline.length) {
        const err = new Error('Aggregate has empty pipeline');
        return cb(err);
      }
  
      prepareDiscriminatorPipeline(this);
  
      model.hooks.execPre('aggregate', this, error => {
        if (error) {
          const _opts = { error: error };
          return model.hooks.execPost('aggregate', this, [null], _opts, error => {
            cb(error);
          });
        }
  
        this.options.explain = true;
  
        model.collection.
          aggregate(this._pipeline, this.options || {}).
          explain((error, result) => {
            const _opts = { error: error };
            return model.hooks.execPost('aggregate', this, [result], _opts, error => {
              if (error) {
                return cb(error);
              }
              return cb(null, result);
            });
          });
      });
    }, model.events);
  };
  
  /**
   * Sets the allowDiskUse option for the aggregation query (ignored for < 2.6.0)
   *
   * ####Example:
   *
   *     await Model.aggregate([{ $match: { foo: 'bar' } }]).allowDiskUse(true);
   *
   * @param {Boolean} value Should tell server it can use hard drive to store data during aggregation.
   * @param {Array} [tags] optional tags for this query
   * @see mongodb http://docs.mongodb.org/manual/reference/command/aggregate/
   */
  
  Aggregate.prototype.allowDiskUse = function(value) {
    this.options.allowDiskUse = value;
    return this;
  };
  
  /**
   * Sets the hint option for the aggregation query (ignored for < 3.6.0)
   *
   * ####Example:
   *
   *     Model.aggregate(..).hint({ qty: 1, category: 1 }).exec(callback)
   *
   * @param {Object|String} value a hint object or the index name
   * @see mongodb http://docs.mongodb.org/manual/reference/command/aggregate/
   */
  
  Aggregate.prototype.hint = function(value) {
    this.options.hint = value;
    return this;
  };
  
  /**
   * Sets the session for this aggregation. Useful for [transactions](/docs/transactions.html).
   *
   * ####Example:
   *
   *     const session = await Model.startSession();
   *     await Model.aggregate(..).session(session);
   *
   * @param {ClientSession} session
   * @see mongodb http://docs.mongodb.org/manual/reference/command/aggregate/
   */
  
  Aggregate.prototype.session = function(session) {
    if (session == null) {
      delete this.options.session;
    } else {
      this.options.session = session;
    }
    return this;
  };
  
  /**
   * Lets you set arbitrary options, for middleware or plugins.
   *
   * ####Example:
   *
   *     const agg = Model.aggregate(..).option({ allowDiskUse: true }); // Set the `allowDiskUse` option
   *     agg.options; // `{ allowDiskUse: true }`
   *
   * @param {Object} options keys to merge into current options
   * @param [options.maxTimeMS] number limits the time this aggregation will run, see [MongoDB docs on `maxTimeMS`](https://docs.mongodb.com/manual/reference/operator/meta/maxTimeMS/)
   * @param [options.allowDiskUse] boolean if true, the MongoDB server will use the hard drive to store data during this aggregation
   * @param [options.collation] object see [`Aggregate.prototype.collation()`](./docs/api.html#aggregate_Aggregate-collation)
   * @param [options.session] ClientSession see [`Aggregate.prototype.session()`](./docs/api.html#aggregate_Aggregate-session)
   * @see mongodb http://docs.mongodb.org/manual/reference/command/aggregate/
   * @return {Aggregate} this
   * @api public
   */
  
  Aggregate.prototype.option = function(value) {
    for (const key in value) {
      this.options[key] = value[key];
    }
    return this;
  };
  
  /**
   * Sets the cursor option option for the aggregation query (ignored for < 2.6.0).
   * Note the different syntax below: .exec() returns a cursor object, and no callback
   * is necessary.
   *
   * ####Example:
   *
   *     const cursor = Model.aggregate(..).cursor({ batchSize: 1000 }).exec();
   *     cursor.eachAsync(function(doc, i) {
   *       // use doc
   *     });
   *
   * @param {Object} options
   * @param {Number} options.batchSize set the cursor batch size
   * @param {Boolean} [options.useMongooseAggCursor] use experimental mongoose-specific aggregation cursor (for `eachAsync()` and other query cursor semantics)
   * @return {Aggregate} this
   * @api public
   * @see mongodb http://mongodb.github.io/node-mongodb-native/2.0/api/AggregationCursor.html
   */
  
  Aggregate.prototype.cursor = function(options) {
    if (!this.options) {
      this.options = {};
    }
    this.options.cursor = options || {};
    return new AggregationCursor(this); // return this;
  };
  
  /**
   * Adds a collation
   *
   * ####Example:
   *
   *     Model.aggregate(..).collation({ locale: 'en_US', strength: 1 }).exec();
   *
   * @param {Object} collation options
   * @return {Aggregate} this
   * @api public
   * @see mongodb http://mongodb.github.io/node-mongodb-native/2.2/api/Collection.html#aggregate
   */
  
  Aggregate.prototype.collation = function(collation) {
    if (!this.options) {
      this.options = {};
    }
    this.options.collation = collation;
    return this;
  };
  
  /**
   * Combines multiple aggregation pipelines.
   *
   * ####Example:
   *
   *     Model.aggregate(...)
   *      .facet({
   *        books: [{ groupBy: '$author' }],
   *        price: [{ $bucketAuto: { groupBy: '$price', buckets: 2 } }]
   *      })
   *      .exec();
   *
   *     // Output: { books: [...], price: [{...}, {...}] }
   *
   * @param {Object} facet options
   * @return {Aggregate} this
   * @see $facet https://docs.mongodb.com/v3.4/reference/operator/aggregation/facet/
   * @api public
   */
  
  Aggregate.prototype.facet = function(options) {
    return this.append({ $facet: options });
  };
  
  /**
   * Helper for [Atlas Text Search](https://docs.atlas.mongodb.com/reference/atlas-search/tutorial/)'s
   * `$search` stage.
   *
   * ####Example:
   *
   *     Model.aggregate().
   *      search({
   *        text: {
   *          query: 'baseball',
   *          path: 'plot'
   *        }
   *      });
   *
   *     // Output: [{ plot: '...', title: '...' }]
   *
   * @param {Object} $search options
   * @return {Aggregate} this
   * @see $search https://docs.atlas.mongodb.com/reference/atlas-search/tutorial/
   * @api public
   */
  
  Aggregate.prototype.search = function(options) {
    return this.append({ $search: options });
  };
  
  /**
   * Returns the current pipeline
   *
   * ####Example:
   *
   *     MyModel.aggregate().match({ test: 1 }).pipeline(); // [{ $match: { test: 1 } }]
   *
   * @return {Array}
   * @api public
   */
  
  
  Aggregate.prototype.pipeline = function() {
    return this._pipeline;
  };
  
  /**
   * Executes the aggregate pipeline on the currently bound Model.
   *
   * ####Example:
   *
   *     aggregate.exec(callback);
   *
   *     // Because a promise is returned, the `callback` is optional.
   *     const promise = aggregate.exec();
   *     promise.then(..);
   *
   * @see Promise #promise_Promise
   * @param {Function} [callback]
   * @return {Promise}
   * @api public
   */
  
  Aggregate.prototype.exec = function(callback) {
    if (!this._model) {
      throw new Error('Aggregate not bound to any Model');
    }
    const model = this._model;
    const collection = this._model.collection;
  
    applyGlobalMaxTimeMS(this.options, model);
  
    if (this.options && this.options.cursor) {
      return new AggregationCursor(this);
    }
  
    return promiseOrCallback(callback, cb => {
      prepareDiscriminatorPipeline(this);
      stringifyFunctionOperators(this._pipeline);
  
      model.hooks.execPre('aggregate', this, error => {
        if (error) {
          const _opts = { error: error };
          return model.hooks.execPost('aggregate', this, [null], _opts, error => {
            cb(error);
          });
        }
        if (!this._pipeline.length) {
          return cb(new Error('Aggregate has empty pipeline'));
        }
  
        const options = utils.clone(this.options || {});
  
        const cursor = collection.aggregate(this._pipeline, options);
  
        if (cursor != null && typeof cursor.then === 'function') {
          return cursor.then(cursor => {
            cursor.toArray((error, result) => {
              const _opts = { error: error };
              model.hooks.execPost('aggregate', this, [result], _opts, (error, result) => {
                if (error) {
                  return cb(error);
                }
  
                cb(null, result);
              });
            });
          }).catch(err => cb(err));
        }
  
        cursor.toArray((error, result) => {
          const _opts = { error: error };
          model.hooks.execPost('aggregate', this, [result], _opts, (error, result) => {
            if (error) {
              return cb(error);
            }
  
            cb(null, result);
          });
        });
      });
    }, model.events);
  };
  
  /**
   * Provides promise for aggregate.
   *
   * ####Example:
   *
   *     Model.aggregate(..).then(successCallback, errorCallback);
   *
   * @see Promise #promise_Promise
   * @param {Function} [resolve] successCallback
   * @param {Function} [reject]  errorCallback
   * @return {Promise}
   */
  Aggregate.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()`](#query_Query-then), but only takes a rejection handler.
   *
   * @param {Function} [reject]
   * @return {Promise}
   * @api public
   */
  
  Aggregate.prototype.catch = function(reject) {
    return this.exec().then(null, reject);
  };
  
  /**
   * Returns an asyncIterator for use with [`for/await/of` loops](https://thecodebarbarian.com/getting-started-with-async-iterators-in-node-js
   * You do not need to call this function explicitly, the JavaScript runtime
   * will call it for you.
   *
   * ####Example
   *
   *     const agg = Model.aggregate([{ $match: { age: { $gte: 25 } } }]);
   *     for await (const doc of agg) {
   *       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 set 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 Aggregate
   * @instance
   * @api public
   */
  
  if (Symbol.asyncIterator != null) {
    Aggregate.prototype[Symbol.asyncIterator] = function() {
      return this.cursor({ useMongooseAggCursor: true }).
        transformNull().
        _transformForAsyncIterator();
    };
  }
  
  /*!
   * Helpers
   */
  
  /**
   * Checks whether an object is likely a pipeline operator
   *
   * @param {Object} obj object to check
   * @return {Boolean}
   * @api private
   */
  
  function isOperator(obj) {
    if (typeof obj !== 'object') {
      return false;
    }
  
    const k = Object.keys(obj);
  
    return k.length === 1 && k.some(key => { return key[0] === '$'; });
  }
  
  /*!
   * Adds the appropriate `$match` pipeline step to the top of an aggregate's
   * pipeline, should it's model is a non-root discriminator type. This is
   * analogous to the `prepareDiscriminatorCriteria` function in `lib/query.js`.
   *
   * @param {Aggregate} aggregate Aggregate to prepare
   */
  
  Aggregate._prepareDiscriminatorPipeline = prepareDiscriminatorPipeline;
  
  function prepareDiscriminatorPipeline(aggregate) {
    const schema = aggregate._model.schema;
    const discriminatorMapping = schema && schema.discriminatorMapping;
  
    if (discriminatorMapping && !discriminatorMapping.isRoot) {
      const originalPipeline = aggregate._pipeline;
      const discriminatorKey = discriminatorMapping.key;
      const discriminatorValue = discriminatorMapping.value;
  
      // If the first pipeline stage is a match and it doesn't specify a `__t`
      // key, add the discriminator key to it. This allows for potential
      // aggregation query optimizations not to be disturbed by this feature.
      if (originalPipeline[0] != null && originalPipeline[0].$match && !originalPipeline[0].$match[discriminatorKey]) {
        originalPipeline[0].$match[discriminatorKey] = discriminatorValue;
        // `originalPipeline` is a ref, so there's no need for
        // aggregate._pipeline = originalPipeline
      } else if (originalPipeline[0] != null && originalPipeline[0].$geoNear) {
        originalPipeline[0].$geoNear.query =
            originalPipeline[0].$geoNear.query || {};
        originalPipeline[0].$geoNear.query[discriminatorKey] = discriminatorValue;
      } else if (originalPipeline[0] != null && originalPipeline[0].$search) {
        if (originalPipeline[1] && originalPipeline[1].$match != null) {
          originalPipeline[1].$match[discriminatorKey] = originalPipeline[1].$match[discriminatorKey] || discriminatorValue;
        } else {
          const match = {};
          match[discriminatorKey] = discriminatorValue;
          originalPipeline.splice(1, 0, { $match: match });
        }
      } else {
        const match = {};
        match[discriminatorKey] = discriminatorValue;
        aggregate._pipeline.unshift({ $match: match });
      }
    }
  }
  
  /*!
   * Exports
   */
  
  module.exports = Aggregate;
  