text: import copy
  import itertools
  import re
  
  import pymongo
  import pymongo.errors
  from bson import SON, json_util
  from bson.code import Code
  from pymongo.collection import ReturnDocument
  from pymongo.common import validate_read_preference
  
  from mongoengine import signals
  from mongoengine.base import get_document
  from mongoengine.common import _import_class
  from mongoengine.connection import get_db
  from mongoengine.context_managers import set_write_concern, switch_db
  from mongoengine.errors import (
      BulkWriteError,
      InvalidQueryError,
      LookUpError,
      NotUniqueError,
      OperationError,
  )
  from mongoengine.queryset import transform
  from mongoengine.queryset.field_list import QueryFieldList
  from mongoengine.queryset.visitor import Q, QNode
  
  __all__ = ("BaseQuerySet", "DO_NOTHING", "NULLIFY", "CASCADE", "DENY", "PULL")
  
  # Delete rules
  DO_NOTHING = 0
  NULLIFY = 1
  CASCADE = 2
  DENY = 3
  PULL = 4
  
  
  class BaseQuerySet:
      """A set of results returned from a query. Wraps a MongoDB cursor,
      providing :class:`~mongoengine.Document` objects as the results.
      """
  
      __dereference = False
      _auto_dereference = True
  
      def __init__(self, document, collection):
          self._document = document
          self._collection_obj = collection
          self._mongo_query = None
          self._query_obj = Q()
          self._cls_query = {}
          self._where_clause = None
          self._loaded_fields = QueryFieldList()
          self._ordering = None
          self._timeout = True
          self._slave_okay = False
          self._read_preference = None
          self._iter = False
          self._scalar = []
          self._none = False
          self._as_pymongo = False
          self._search_text = None
  
          # If inheritance is allowed, only return instances and instances of
          # subclasses of the class being used
          if document._meta.get("allow_inheritance") is True:
              if len(self._document._subclasses) == 1:
                  self._cls_query = {"_cls": self._document._subclasses[0]}
              else:
                  self._cls_query = {"_cls": {"$in": self._document._subclasses}}
              self._loaded_fields = QueryFieldList(always_include=["_cls"])
  
          self._cursor_obj = None
          self._limit = None
          self._skip = None
          self._hint = -1  # Using -1 as None is a valid value for hint
          self._collation = None
          self._batch_size = None
          self.only_fields = []
          self._max_time_ms = None
          self._comment = None
  
      def __call__(self, q_obj=None, **query):
          """Filter the selected documents by calling the
          :class:`~mongoengine.queryset.QuerySet` with a query.
  
          :param q_obj: a :class:`~mongoengine.queryset.Q` object to be used in
              the query; the :class:`~mongoengine.queryset.QuerySet` is filtered
              multiple times with different :class:`~mongoengine.queryset.Q`
              objects, only the last one will be used.
          :param query: Django-style query keyword arguments.
          """
          query = Q(**query)
          if q_obj:
              # Make sure proper query object is passed.
              if not isinstance(q_obj, QNode):
                  msg = (
                      "Not a query object: %s. "
                      "Did you intend to use key=value?" % q_obj
                  )
                  raise InvalidQueryError(msg)
              query &= q_obj
  
          queryset = self.clone()
          queryset._query_obj &= query
          queryset._mongo_query = None
          queryset._cursor_obj = None
  
          return queryset
  
      def __getstate__(self):
          """
          Need for pickling queryset
  
          See https://github.com/MongoEngine/mongoengine/issues/442
          """
  
          obj_dict = self.__dict__.copy()
  
          # don't picke collection, instead pickle collection params
          obj_dict.pop("_collection_obj")
  
          # don't pickle cursor
          obj_dict["_cursor_obj"] = None
  
          return obj_dict
  
      def __setstate__(self, obj_dict):
          """
          Need for pickling queryset
  
          See https://github.com/MongoEngine/mongoengine/issues/442
          """
  
          obj_dict["_collection_obj"] = obj_dict["_document"]._get_collection()
  
          # update attributes
          self.__dict__.update(obj_dict)
  
          # forse load cursor
          # self._cursor
  
      def __getitem__(self, key):
          """Return a document instance corresponding to a given index if
          the key is an integer. If the key is a slice, translate its
          bounds into a skip and a limit, and return a cloned queryset
          with that skip/limit applied. For example:
  
          >>> User.objects[0]
          <User: User object>
          >>> User.objects[1:3]
          [<User: User object>, <User: User object>]
          """
          queryset = self.clone()
  
          # Handle a slice
          if isinstance(key, slice):
              queryset._cursor_obj = queryset._cursor[key]
              queryset._skip, queryset._limit = key.start, key.stop
              if key.start and key.stop:
                  queryset._limit = key.stop - key.start
  
              # Allow further QuerySet modifications to be performed
              return queryset
  
          # Handle an index
          elif isinstance(key, int):
              if queryset._scalar:
                  return queryset._get_scalar(
                      queryset._document._from_son(
                          queryset._cursor[key],
                          _auto_dereference=self._auto_dereference,
                          only_fields=self.only_fields,
                      )
                  )
  
              if queryset._as_pymongo:
                  return queryset._cursor[key]
  
              return queryset._document._from_son(
                  queryset._cursor[key],
                  _auto_dereference=self._auto_dereference,
                  only_fields=self.only_fields,
              )
  
          raise TypeError("Provide a slice or an integer index")
  
      def __iter__(self):
          raise NotImplementedError
  
      def _has_data(self):
          """Return True if cursor has any data."""
          queryset = self.order_by()
          return False if queryset.first() is None else True
  
      def __bool__(self):
          """Avoid to open all records in an if stmt in Py3."""
          return self._has_data()
  
      __nonzero__ = __bool__  # For Py2 support
  
      # Core functions
  
      def all(self):
          """Returns a copy of the current QuerySet."""
          return self.__call__()
  
      def filter(self, *q_objs, **query):
          """An alias of :meth:`~mongoengine.queryset.QuerySet.__call__`"""
          return self.__call__(*q_objs, **query)
  
      def search_text(self, text, language=None):
          """
          Start a text search, using text indexes.
          Require: MongoDB server version 2.6+.
  
          :param language:  The language that determines the list of stop words
              for the search and the rules for the stemmer and tokenizer.
              If not specified, the search uses the default language of the index.
              For supported languages, see
              `Text Search Languages <http://docs.mongodb.org/manual/reference/text-search-languages/#text-search-languages>`.
          """
          queryset = self.clone()
          if queryset._search_text:
              raise OperationError("It is not possible to use search_text two times.")
  
          query_kwargs = SON({"$search": text})
          if language:
              query_kwargs["$language"] = language
  
          queryset._query_obj &= Q(__raw__={"$text": query_kwargs})
          queryset._mongo_query = None
          queryset._cursor_obj = None
          queryset._search_text = text
  
          return queryset
  
      def get(self, *q_objs, **query):
          """Retrieve the the matching object raising
          :class:`~mongoengine.queryset.MultipleObjectsReturned` or
          `DocumentName.MultipleObjectsReturned` exception if multiple results
          and :class:`~mongoengine.queryset.DoesNotExist` or
          `DocumentName.DoesNotExist` if no results are found.
  
          .. versionadded:: 0.3
          """
          queryset = self.clone()
          queryset = queryset.order_by().limit(2)
          queryset = queryset.filter(*q_objs, **query)
  
          try:
              result = next(queryset)
          except StopIteration:
              msg = "%s matching query does not exist." % queryset._document._class_name
              raise queryset._document.DoesNotExist(msg)
          try:
              next(queryset)
          except StopIteration:
              return result
  
          # If we were able to retrieve the 2nd doc, rewind the cursor and
          # raise the MultipleObjectsReturned exception.
          queryset.rewind()
          message = "%d items returned, instead of 1" % queryset.count()
          raise queryset._document.MultipleObjectsReturned(message)
  
      def create(self, **kwargs):
          """Create new object. Returns the saved object instance.
  
          .. versionadded:: 0.4
          """
          return self._document(**kwargs).save(force_insert=True)
  
      def first(self):
          """Retrieve the first object matching the query."""
          queryset = self.clone()
          try:
              result = queryset[0]
          except IndexError:
              result = None
          return result
  
      def insert(
          self, doc_or_docs, load_bulk=True, write_concern=None, signal_kwargs=None
      ):
          """bulk insert documents
  
          :param doc_or_docs: a document or list of documents to be inserted
          :param load_bulk (optional): If True returns the list of document
              instances
          :param write_concern: Extra keyword arguments are passed down to
                  :meth:`~pymongo.collection.Collection.insert`
                  which will be used as options for the resultant
                  ``getLastError`` command.  For example,
                  ``insert(..., {w: 2, fsync: True})`` will wait until at least
                  two servers have recorded the write and will force an fsync on
                  each server being written to.
          :parm signal_kwargs: (optional) kwargs dictionary to be passed to
              the signal calls.
  
          By default returns document instances, set ``load_bulk`` to False to
          return just ``ObjectIds``
  
          .. versionadded:: 0.5
          .. versionchanged:: 0.10.7
              Add signal_kwargs argument
          """
          Document = _import_class("Document")
  
          if write_concern is None:
              write_concern = {}
  
          docs = doc_or_docs
          return_one = False
          if isinstance(docs, Document) or issubclass(docs.__class__, Document):
              return_one = True
              docs = [docs]
  
          for doc in docs:
              if not isinstance(doc, self._document):
                  msg = "Some documents inserted aren't instances of %s" % str(
                      self._document
                  )
                  raise OperationError(msg)
              if doc.pk and not doc._created:
                  msg = "Some documents have ObjectIds, use doc.update() instead"
                  raise OperationError(msg)
  
          signal_kwargs = signal_kwargs or {}
          signals.pre_bulk_insert.send(self._document, documents=docs, **signal_kwargs)
  
          raw = [doc.to_mongo() for doc in docs]
  
          with set_write_concern(self._collection, write_concern) as collection:
              insert_func = collection.insert_many
              if return_one:
                  raw = raw[0]
                  insert_func = collection.insert_one
  
          try:
              inserted_result = insert_func(raw)
              ids = (
                  [inserted_result.inserted_id]
                  if return_one
                  else inserted_result.inserted_ids
              )
          except pymongo.errors.DuplicateKeyError as err:
              message = "Could not save document (%s)"
              raise NotUniqueError(message % str(err))
          except pymongo.errors.BulkWriteError as err:
              # inserting documents that already have an _id field will
              # give huge performance debt or raise
              message = "Bulk write error: (%s)"
              raise BulkWriteError(message % str(err.details))
          except pymongo.errors.OperationFailure as err:
              message = "Could not save document (%s)"
              if re.match("^E1100[01] duplicate key", str(err)):
                  # E11000 - duplicate key error index
                  # E11001 - duplicate key on update
                  message = "Tried to save duplicate unique keys (%s)"
                  raise NotUniqueError(message % str(err))
              raise OperationError(message % str(err))
  
          # Apply inserted_ids to documents
          for doc, doc_id in zip(docs, ids):
              doc.pk = doc_id
  
          if not load_bulk:
              signals.post_bulk_insert.send(
                  self._document, documents=docs, loaded=False, **signal_kwargs
              )
              return ids[0] if return_one else ids
  
          documents = self.in_bulk(ids)
          results = [documents.get(obj_id) for obj_id in ids]
          signals.post_bulk_insert.send(
              self._document, documents=results, loaded=True, **signal_kwargs
          )
          return results[0] if return_one else results
  
      def count(self, with_limit_and_skip=False):
          """Count the selected elements in the query.
  
          :param with_limit_and_skip (optional): take any :meth:`limit` or
              :meth:`skip` that has been applied to this cursor into account when
              getting the count
          """
          if self._limit == 0 and with_limit_and_skip is False or self._none:
              return 0
          count = self._cursor.count(with_limit_and_skip=with_limit_and_skip)
          self._cursor_obj = None
          return count
  
      def delete(self, write_concern=None, _from_doc_delete=False, cascade_refs=None):
          """Delete the documents matched by the query.
  
          :param write_concern: Extra keyword arguments are passed down which
              will be used as options for the resultant
              ``getLastError`` command.  For example,
              ``save(..., write_concern={w: 2, fsync: True}, ...)`` will
              wait until at least two servers have recorded the write and
              will force an fsync on the primary server.
          :param _from_doc_delete: True when called from document delete therefore
              signals will have been triggered so don't loop.
  
          :returns number of deleted documents
          """
          queryset = self.clone()
          doc = queryset._document
  
          if write_concern is None:
              write_concern = {}
  
          # Handle deletes where skips or limits have been applied or
          # there is an untriggered delete signal
          has_delete_signal = signals.signals_available and (
              signals.pre_delete.has_receivers_for(doc)
              or signals.post_delete.has_receivers_for(doc)
          )
  
          call_document_delete = (
              queryset._skip or queryset._limit or has_delete_signal
          ) and not _from_doc_delete
  
          if call_document_delete:
              cnt = 0
              for doc in queryset:
                  doc.delete(**write_concern)
                  cnt += 1
              return cnt
  
          delete_rules = doc._meta.get("delete_rules") or {}
          delete_rules = list(delete_rules.items())
  
          # Check for DENY rules before actually deleting/nullifying any other
          # references
          for rule_entry, rule in delete_rules:
              document_cls, field_name = rule_entry
              if document_cls._meta.get("abstract"):
                  continue
  
              if rule == DENY:
                  refs = document_cls.objects(**{field_name + "__in": self})
                  if refs.limit(1).count() > 0:
                      raise OperationError(
                          "Could not delete document (%s.%s refers to it)"
                          % (document_cls.__name__, field_name)
                      )
  
          # Check all the other rules
          for rule_entry, rule in delete_rules:
              document_cls, field_name = rule_entry
              if document_cls._meta.get("abstract"):
                  continue
  
              if rule == CASCADE:
                  cascade_refs = set() if cascade_refs is None else cascade_refs
                  # Handle recursive reference
                  if doc._collection == document_cls._collection:
                      for ref in queryset:
                          cascade_refs.add(ref.id)
                  refs = document_cls.objects(
                      **{field_name + "__in": self, "pk__nin": cascade_refs}
                  )
                  if refs.count() > 0:
                      refs.delete(write_concern=write_concern, cascade_refs=cascade_refs)
              elif rule == NULLIFY:
                  document_cls.objects(**{field_name + "__in": self}).update(
                      write_concern=write_concern, **{"unset__%s" % field_name: 1}
                  )
              elif rule == PULL:
                  document_cls.objects(**{field_name + "__in": self}).update(
                      write_concern=write_concern, **{"pull_all__%s" % field_name: self}
                  )
  
          with set_write_concern(queryset._collection, write_concern) as collection:
              result = collection.delete_many(queryset._query)
  
              # If we're using an unack'd write concern, we don't really know how
              # many items have been deleted at this point, hence we only return
              # the count for ack'd ops.
              if result.acknowledged:
                  return result.deleted_count
  
      def update(
          self, upsert=False, multi=True, write_concern=None, full_result=False, **update
      ):
          """Perform an atomic update on the fields matched by the query.
  
          :param upsert: insert if document doesn't exist (default ``False``)
          :param multi: Update multiple documents.
          :param write_concern: Extra keyword arguments are passed down which
              will be used as options for the resultant
              ``getLastError`` command.  For example,
              ``save(..., write_concern={w: 2, fsync: True}, ...)`` will
              wait until at least two servers have recorded the write and
              will force an fsync on the primary server.
          :param full_result: Return the associated ``pymongo.UpdateResult`` rather than just the number
              updated items
          :param update: Django-style update keyword arguments
  
          :returns the number of updated documents (unless ``full_result`` is True)
  
          .. versionadded:: 0.2
          """
          if not update and not upsert:
              raise OperationError("No update parameters, would remove data")
  
          if write_concern is None:
              write_concern = {}
  
          queryset = self.clone()
          query = queryset._query
          update = transform.update(queryset._document, **update)
  
          # If doing an atomic upsert on an inheritable class
          # then ensure we add _cls to the update operation
          if upsert and "_cls" in query:
              if "$set" in update:
                  update["$set"]["_cls"] = queryset._document._class_name
              else:
                  update["$set"] = {"_cls": queryset._document._class_name}
          try:
              with set_write_concern(queryset._collection, write_concern) as collection:
                  update_func = collection.update_one
                  if multi:
                      update_func = collection.update_many
                  result = update_func(query, update, upsert=upsert)
              if full_result:
                  return result
              elif result.raw_result:
                  return result.raw_result["n"]
          except pymongo.errors.DuplicateKeyError as err:
              raise NotUniqueError("Update failed (%s)" % str(err))
          except pymongo.errors.OperationFailure as err:
              if str(err) == "multi not coded yet":
                  message = "update() method requires MongoDB 1.1.3+"
                  raise OperationError(message)
              raise OperationError("Update failed (%s)" % str(err))
  
      def upsert_one(self, write_concern=None, **update):
          """Overwrite or add the first document matched by the query.
  
          :param write_concern: Extra keyword arguments are passed down which
              will be used as options for the resultant
              ``getLastError`` command.  For example,
              ``save(..., write_concern={w: 2, fsync: True}, ...)`` will
              wait until at least two servers have recorded the write and
              will force an fsync on the primary server.
          :param update: Django-style update keyword arguments
  
          :returns the new or overwritten document
  
          .. versionadded:: 0.10.2
          """
  
          atomic_update = self.update(
              multi=False,
              upsert=True,
              write_concern=write_concern,
              full_result=True,
              **update
          )
  
          if atomic_update.raw_result["updatedExisting"]:
              document = self.get()
          else:
              document = self._document.objects.with_id(atomic_update.upserted_id)
          return document
  
      def update_one(self, upsert=False, write_concern=None, full_result=False, **update):
          """Perform an atomic update on the fields of the first document
          matched by the query.
  
          :param upsert: insert if document doesn't exist (default ``False``)
          :param write_concern: Extra keyword arguments are passed down which
              will be used as options for the resultant
              ``getLastError`` command.  For example,
              ``save(..., write_concern={w: 2, fsync: True}, ...)`` will
              wait until at least two servers have recorded the write and
              will force an fsync on the primary server.
          :param full_result: Return the associated ``pymongo.UpdateResult`` rather than just the number
              updated items
          :param update: Django-style update keyword arguments
              full_result
          :returns the number of updated documents (unless ``full_result`` is True)
          .. versionadded:: 0.2
          """
          return self.update(
              upsert=upsert,
              multi=False,
              write_concern=write_concern,
              full_result=full_result,
              **update
          )
  
      def modify(self, upsert=False, remove=False, new=False, **update):
          """Update and return the updated document.
  
          Returns either the document before or after modification based on `new`
          parameter. If no documents match the query and `upsert` is false,
          returns ``None``. If upserting and `new` is false, returns ``None``.
  
          :param upsert: insert if document doesn't exist (default ``False``)
          :param remove: remove rather than updating (default ``False``)
          :param new: return updated rather than original document
              (default ``False``)
          :param update: Django-style update keyword arguments
  
          .. versionadded:: 0.9
          """
  
          if remove and new:
              raise OperationError("Conflicting parameters: remove and new")
  
          if not update and not upsert and not remove:
              raise OperationError("No update parameters, must either update or remove")
  
          queryset = self.clone()
          query = queryset._query
          if not remove:
              update = transform.update(queryset._document, **update)
          sort = queryset._ordering
  
          try:
              if remove:
                  result = queryset._collection.find_one_and_delete(
                      query, sort=sort, **self._cursor_args
                  )
              else:
                  if new:
                      return_doc = ReturnDocument.AFTER
                  else:
                      return_doc = ReturnDocument.BEFORE
                  result = queryset._collection.find_one_and_update(
                      query,
                      update,
                      upsert=upsert,
                      sort=sort,
                      return_document=return_doc,
                      **self._cursor_args
                  )
          except pymongo.errors.DuplicateKeyError as err:
              raise NotUniqueError("Update failed (%s)" % err)
          except pymongo.errors.OperationFailure as err:
              raise OperationError("Update failed (%s)" % err)
          else:
              if result is not None:
                  result = self._document._from_son(result, only_fields=self.only_fields)
  
          return result
  
      def with_id(self, object_id):
          """Retrieve the object matching the id provided.  Uses `object_id` only
          and raises InvalidQueryError if a filter has been applied. Returns
          `None` if no document exists with that id.
  
          :param object_id: the value for the id of the document to look up
  
          .. versionchanged:: 0.6 Raises InvalidQueryError if filter has been set
          """
          queryset = self.clone()
          if not queryset._query_obj.empty:
              msg = "Cannot use a filter whilst using `with_id`"
              raise InvalidQueryError(msg)
          return queryset.filter(pk=object_id).first()
  
      def in_bulk(self, object_ids):
          r"""Retrieve a set of documents by their ids.
  
          :param object_ids: a list or tuple of ``ObjectId``\ s
          :rtype: dict of ObjectIds as keys and collection-specific
                  Document subclasses as values.
  
          .. versionadded:: 0.3
          """
          doc_map = {}
  
          docs = self._collection.find({"_id": {"$in": object_ids}}, **self._cursor_args)
          if self._scalar:
              for doc in docs:
                  doc_map[doc["_id"]] = self._get_scalar(
                      self._document._from_son(doc, only_fields=self.only_fields)
                  )
          elif self._as_pymongo:
              for doc in docs:
                  doc_map[doc["_id"]] = doc
          else:
              for doc in docs:
                  doc_map[doc["_id"]] = self._document._from_son(
                      doc,
                      only_fields=self.only_fields,
                      _auto_dereference=self._auto_dereference,
                  )
  
          return doc_map
  
      def none(self):
          """Helper that just returns a list"""
          queryset = self.clone()
          queryset._none = True
          return queryset
  
      def no_sub_classes(self):
          """Filter for only the instances of this specific document.
  
          Do NOT return any inherited documents.
          """
          if self._document._meta.get("allow_inheritance") is True:
              self._cls_query = {"_cls": self._document._class_name}
  
          return self
  
      def using(self, alias):
          """This method is for controlling which database the QuerySet will be
          evaluated against if you are using more than one database.
  
          :param alias: The database alias
  
          .. versionadded:: 0.9
          """
  
          with switch_db(self._document, alias) as cls:
              collection = cls._get_collection()
  
          return self._clone_into(self.__class__(self._document, collection))
  
      def clone(self):
          """Create a copy of the current queryset."""
          return self._clone_into(self.__class__(self._document, self._collection_obj))
  
      def _clone_into(self, new_qs):
          """Copy all of the relevant properties of this queryset to
          a new queryset (which has to be an instance of
          :class:`~mongoengine.queryset.base.BaseQuerySet`).
          """
          if not isinstance(new_qs, BaseQuerySet):
              raise OperationError(
                  "%s is not a subclass of BaseQuerySet" % new_qs.__name__
              )
  
          copy_props = (
              "_mongo_query",
              "_cls_query",
              "_none",
              "_query_obj",
              "_where_clause",
              "_loaded_fields",
              "_ordering",
              "_timeout",
              "_slave_okay",
              "_read_preference",
              "_iter",
              "_scalar",
              "_as_pymongo",
              "_limit",
              "_skip",
              "_hint",
              "_collation",
              "_auto_dereference",
              "_search_text",
              "only_fields",
              "_max_time_ms",
              "_comment",
              "_batch_size",
          )
  
          for prop in copy_props:
              val = getattr(self, prop)
              setattr(new_qs, prop, copy.copy(val))
  
          if self._cursor_obj:
              new_qs._cursor_obj = self._cursor_obj.clone()
  
          return new_qs
  
      def select_related(self, max_depth=1):
          """Handles dereferencing of :class:`~bson.dbref.DBRef` objects or
          :class:`~bson.object_id.ObjectId` a maximum depth in order to cut down
          the number queries to mongodb.
  
          .. versionadded:: 0.5
          """
          # Make select related work the same for querysets
          max_depth += 1
          queryset = self.clone()
          return queryset._dereference(queryset, max_depth=max_depth)
  
      def limit(self, n):
          """Limit the number of returned documents to `n`. This may also be
          achieved using array-slicing syntax (e.g. ``User.objects[:5]``).
  
          :param n: the maximum number of objects to return if n is greater than 0.
          When 0 is passed, returns all the documents in the cursor
          """
          queryset = self.clone()
          queryset._limit = n
  
          # If a cursor object has already been created, apply the limit to it.
          if queryset._cursor_obj:
              queryset._cursor_obj.limit(queryset._limit)
  
          return queryset
  
      def skip(self, n):
          """Skip `n` documents before returning the results. This may also be
          achieved using array-slicing syntax (e.g. ``User.objects[5:]``).
  
          :param n: the number of objects to skip before returning results
          """
          queryset = self.clone()
          queryset._skip = n
  
          # If a cursor object has already been created, apply the skip to it.
          if queryset._cursor_obj:
              queryset._cursor_obj.skip(queryset._skip)
  
          return queryset
  
      def hint(self, index=None):
          """Added 'hint' support, telling Mongo the proper index to use for the
          query.
  
          Judicious use of hints can greatly improve query performance. When
          doing a query on multiple fields (at least one of which is indexed)
          pass the indexed field as a hint to the query.
  
          Hinting will not do anything if the corresponding index does not exist.
          The last hint applied to this cursor takes precedence over all others.
  
          .. versionadded:: 0.5
          """
          queryset = self.clone()
          queryset._hint = index
  
          # If a cursor object has already been created, apply the hint to it.
          if queryset._cursor_obj:
              queryset._cursor_obj.hint(queryset._hint)
  
          return queryset
  
      def collation(self, collation=None):
          """
          Collation allows users to specify language-specific rules for string
          comparison, such as rules for lettercase and accent marks.
          :param collation: `~pymongo.collation.Collation` or dict with
          following fields:
              {
                  locale: str,
                  caseLevel: bool,
                  caseFirst: str,
                  strength: int,
                  numericOrdering: bool,
                  alternate: str,
                  maxVariable: str,
                  backwards: str
              }
          Collation should be added to indexes like in test example
          """
          queryset = self.clone()
          queryset._collation = collation
  
          if queryset._cursor_obj:
              queryset._cursor_obj.collation(collation)
  
          return queryset
  
      def batch_size(self, size):
          """Limit the number of documents returned in a single batch (each
          batch requires a round trip to the server).
  
          See http://api.mongodb.com/python/current/api/pymongo/cursor.html#pymongo.cursor.Cursor.batch_size
          for details.
  
          :param size: desired size of each batch.
          """
          queryset = self.clone()
          queryset._batch_size = size
  
          # If a cursor object has already been created, apply the batch size to it.
          if queryset._cursor_obj:
              queryset._cursor_obj.batch_size(queryset._batch_size)
  
          return queryset
  
      def distinct(self, field):
          """Return a list of distinct values for a given field.
  
          :param field: the field to select distinct values from
  
          .. note:: This is a command and won't take ordering or limit into
             account.
  
          .. versionadded:: 0.4
          .. versionchanged:: 0.5 - Fixed handling references
          .. versionchanged:: 0.6 - Improved db_field refrence handling
          """
          queryset = self.clone()
  
          try:
              field = self._fields_to_dbfields([field]).pop()
          except LookUpError:
              pass
  
          distinct = self._dereference(
              queryset._cursor.distinct(field), 1, name=field, instance=self._document
          )
  
          doc_field = self._document._fields.get(field.split(".", 1)[0])
          instance = None
  
          # We may need to cast to the correct type eg. ListField(EmbeddedDocumentField)
          EmbeddedDocumentField = _import_class("EmbeddedDocumentField")
          ListField = _import_class("ListField")
          GenericEmbeddedDocumentField = _import_class("GenericEmbeddedDocumentField")
          if isinstance(doc_field, ListField):
              doc_field = getattr(doc_field, "field", doc_field)
          if isinstance(doc_field, (EmbeddedDocumentField, GenericEmbeddedDocumentField)):
              instance = getattr(doc_field, "document_type", None)
  
          # handle distinct on subdocuments
          if "." in field:
              for field_part in field.split(".")[1:]:
                  # if looping on embedded document, get the document type instance
                  if instance and isinstance(
                      doc_field, (EmbeddedDocumentField, GenericEmbeddedDocumentField)
                  ):
                      doc_field = instance
                  # now get the subdocument
                  doc_field = getattr(doc_field, field_part, doc_field)
                  # We may need to cast to the correct type eg. ListField(EmbeddedDocumentField)
                  if isinstance(doc_field, ListField):
                      doc_field = getattr(doc_field, "field", doc_field)
                  if isinstance(
                      doc_field, (EmbeddedDocumentField, GenericEmbeddedDocumentField)
                  ):
                      instance = getattr(doc_field, "document_type", None)
  
          if instance and isinstance(
              doc_field, (EmbeddedDocumentField, GenericEmbeddedDocumentField)
          ):
              distinct = [instance(**doc) for doc in distinct]
  
          return distinct
  
      def only(self, *fields):
          """Load only a subset of this document's fields. ::
  
              post = BlogPost.objects(...).only('title', 'author.name')
  
          .. note :: `only()` is chainable and will perform a union ::
              So with the following it will fetch both: `title` and `author.name`::
  
                  post = BlogPost.objects.only('title').only('author.name')
  
          :func:`~mongoengine.queryset.QuerySet.all_fields` will reset any
          field filters.
  
          :param fields: fields to include
  
          .. versionadded:: 0.3
          .. versionchanged:: 0.5 - Added subfield support
          """
          fields = {f: QueryFieldList.ONLY for f in fields}
          self.only_fields = list(fields.keys())
          return self.fields(True, **fields)
  
      def exclude(self, *fields):
          """Opposite to .only(), exclude some document's fields. ::
  
              post = BlogPost.objects(...).exclude('comments')
  
          .. note :: `exclude()` is chainable and will perform a union ::
              So with the following it will exclude both: `title` and `author.name`::
  
                  post = BlogPost.objects.exclude('title').exclude('author.name')
  
          :func:`~mongoengine.queryset.QuerySet.all_fields` will reset any
          field filters.
  
          :param fields: fields to exclude
  
          .. versionadded:: 0.5
          """
          fields = {f: QueryFieldList.EXCLUDE for f in fields}
          return self.fields(**fields)
  
      def fields(self, _only_called=False, **kwargs):
          """Manipulate how you load this document's fields. Used by `.only()`
          and `.exclude()` to manipulate which fields to retrieve. If called
          directly, use a set of kwargs similar to the MongoDB projection
          document. For example:
  
          Include only a subset of fields:
  
              posts = BlogPost.objects(...).fields(author=1, title=1)
  
          Exclude a specific field:
  
              posts = BlogPost.objects(...).fields(comments=0)
  
          To retrieve a subrange of array elements:
  
              posts = BlogPost.objects(...).fields(slice__comments=5)
  
          :param kwargs: A set of keyword arguments identifying what to
              include, exclude, or slice.
  
          .. versionadded:: 0.5
          """
  
          # Check for an operator and transform to mongo-style if there is
          operators = ["slice"]
          cleaned_fields = []
          for key, value in kwargs.items():
              parts = key.split("__")
              if parts[0] in operators:
                  op = parts.pop(0)
                  value = {"$" + op: value}
              key = ".".join(parts)
              cleaned_fields.append((key, value))
  
          # Sort fields by their values, explicitly excluded fields first, then
          # explicitly included, and then more complicated operators such as
          # $slice.
          def _sort_key(field_tuple):
              _, value = field_tuple
              if isinstance(value, int):
                  return value  # 0 for exclusion, 1 for inclusion
              return 2  # so that complex values appear last
  
          fields = sorted(cleaned_fields, key=_sort_key)
  
          # Clone the queryset, group all fields by their value, convert
          # each of them to db_fields, and set the queryset's _loaded_fields
          queryset = self.clone()
          for value, group in itertools.groupby(fields, lambda x: x[1]):
              fields = [field for field, value in group]
              fields = queryset._fields_to_dbfields(fields)
              queryset._loaded_fields += QueryFieldList(
                  fields, value=value, _only_called=_only_called
              )
  
          return queryset
  
      def all_fields(self):
          """Include all fields. Reset all previously calls of .only() or
          .exclude(). ::
  
              post = BlogPost.objects.exclude('comments').all_fields()
  
          .. versionadded:: 0.5
          """
          queryset = self.clone()
          queryset._loaded_fields = QueryFieldList(
              always_include=queryset._loaded_fields.always_include
          )
          return queryset
  
      def order_by(self, *keys):
          """Order the :class:`~mongoengine.queryset.QuerySet` by the given keys.
  
          The order may be specified by prepending each of the keys by a "+" or
          a "-". Ascending order is assumed if there's no prefix.
  
          If no keys are passed, existing ordering is cleared instead.
  
          :param keys: fields to order the query results by; keys may be
              prefixed with "+" or a "-" to determine the ordering direction.
          """
          queryset = self.clone()
  
          old_ordering = queryset._ordering
          new_ordering = queryset._get_order_by(keys)
  
          if queryset._cursor_obj:
  
              # If a cursor object has already been created, apply the sort to it
              if new_ordering:
                  queryset._cursor_obj.sort(new_ordering)
  
              # If we're trying to clear a previous explicit ordering, we need
              # to clear the cursor entirely (because PyMongo doesn't allow
              # clearing an existing sort on a cursor).
              elif old_ordering:
                  queryset._cursor_obj = None
  
          queryset._ordering = new_ordering
  
          return queryset
  
      def clear_cls_query(self):
          """Clear the default "_cls" query.
  
          By default, all queries generated for documents that allow inheritance
          include an extra "_cls" clause. In most cases this is desirable, but
          sometimes you might achieve better performance if you clear that
          default query.
  
          Scan the code for `_cls_query` to get more details.
          """
          queryset = self.clone()
          queryset._cls_query = {}
          return queryset
  
      def comment(self, text):
          """Add a comment to the query.
  
          See https://docs.mongodb.com/manual/reference/method/cursor.comment/#cursor.comment
          for details.
          """
          return self._chainable_method("comment", text)
  
      def explain(self):
          r"""Return an explain plan record for the
          :class:`~mongoengine.queryset.QuerySet`\ 's cursor.
          """
          return self._cursor.explain()
  
      def timeout(self, enabled):
          """Enable or disable the default mongod timeout when querying. (no_cursor_timeout option)
  
          :param enabled: whether or not the timeout is used
  
          ..versionchanged:: 0.5 - made chainable
          """
          queryset = self.clone()
          queryset._timeout = enabled
          return queryset
  
      def read_preference(self, read_preference):
          """Change the read_preference when querying.
  
          :param read_preference: override ReplicaSetConnection-level
              preference.
          """
          validate_read_preference("read_preference", read_preference)
          queryset = self.clone()
          queryset._read_preference = read_preference
          queryset._cursor_obj = None  # we need to re-create the cursor object whenever we apply read_preference
          return queryset
  
      def scalar(self, *fields):
          """Instead of returning Document instances, return either a specific
          value or a tuple of values in order.
  
          Can be used along with
          :func:`~mongoengine.queryset.QuerySet.no_dereference` to turn off
          dereferencing.
  
          .. note:: This effects all results and can be unset by calling
                    ``scalar`` without arguments. Calls ``only`` automatically.
  
          :param fields: One or more fields to return instead of a Document.
          """
          queryset = self.clone()
          queryset._scalar = list(fields)
  
          if fields:
              queryset = queryset.only(*fields)
          else:
              queryset = queryset.all_fields()
  
          return queryset
  
      def values_list(self, *fields):
          """An alias for scalar"""
          return self.scalar(*fields)
  
      def as_pymongo(self):
          """Instead of returning Document instances, return raw values from
          pymongo.
  
          This method is particularly useful if you don't need dereferencing
          and care primarily about the speed of data retrieval.
          """
          queryset = self.clone()
          queryset._as_pymongo = True
          return queryset
  
      def max_time_ms(self, ms):
          """Wait `ms` milliseconds before killing the query on the server
  
          :param ms: the number of milliseconds before killing the query on the server
          """
          return self._chainable_method("max_time_ms", ms)
  
      # JSON Helpers
  
      def to_json(self, *args, **kwargs):
          """Converts a queryset to JSON"""
          return json_util.dumps(self.as_pymongo(), *args, **kwargs)
  
      def from_json(self, json_data):
          """Converts json data to unsaved objects"""
          son_data = json_util.loads(json_data)
          return [
              self._document._from_son(data, only_fields=self.only_fields)
              for data in son_data
          ]
  
      def aggregate(self, *pipeline, **kwargs):
          """
          Perform a aggregate function based in your queryset params
          :param pipeline: list of aggregation commands,\
              see: http://docs.mongodb.org/manual/core/aggregation-pipeline/
  
          .. versionadded:: 0.9
          """
          initial_pipeline = []
  
          if self._query:
              initial_pipeline.append({"$match": self._query})
  
          if self._ordering:
              initial_pipeline.append({"$sort": dict(self._ordering)})
  
          if self._limit is not None:
              # As per MongoDB Documentation (https://docs.mongodb.com/manual/reference/operator/aggregation/limit/),
              # keeping limit stage right after sort stage is more efficient. But this leads to wrong set of documents
              # for a skip stage that might succeed these. So we need to maintain more documents in memory in such a
              # case (https://stackoverflow.com/a/24161461).
              initial_pipeline.append({"$limit": self._limit + (self._skip or 0)})
  
          if self._skip is not None:
              initial_pipeline.append({"$skip": self._skip})
  
          pipeline = initial_pipeline + list(pipeline)
  
          if self._read_preference is not None:
              return self._collection.with_options(
                  read_preference=self._read_preference
              ).aggregate(pipeline, cursor={}, **kwargs)
  
          return self._collection.aggregate(pipeline, cursor={}, **kwargs)
  
      # JS functionality
      def map_reduce(
          self, map_f, reduce_f, output, finalize_f=None, limit=None, scope=None
      ):
          """Perform a map/reduce query using the current query spec
          and ordering. While ``map_reduce`` respects ``QuerySet`` chaining,
          it must be the last call made, as it does not return a maleable
          ``QuerySet``.
  
          See the :meth:`~mongoengine.tests.QuerySetTest.test_map_reduce`
          and :meth:`~mongoengine.tests.QuerySetTest.test_map_advanced`
          tests in ``tests.queryset.QuerySetTest`` for usage examples.
  
          :param map_f: map function, as :class:`~bson.code.Code` or string
          :param reduce_f: reduce function, as
                           :class:`~bson.code.Code` or string
          :param output: output collection name, if set to 'inline' will try to
             use :class:`~pymongo.collection.Collection.inline_map_reduce`
             This can also be a dictionary containing output options
             see: http://docs.mongodb.org/manual/reference/command/mapReduce/#dbcmd.mapReduce
          :param finalize_f: finalize function, an optional function that
                             performs any post-reduction processing.
          :param scope: values to insert into map/reduce global scope. Optional.
          :param limit: number of objects from current query to provide
                        to map/reduce method
  
          Returns an iterator yielding
          :class:`~mongoengine.document.MapReduceDocument`.
  
          .. note::
  
              Map/Reduce changed in server version **>= 1.7.4**. The PyMongo
              :meth:`~pymongo.collection.Collection.map_reduce` helper requires
              PyMongo version **>= 1.11**.
  
          .. versionchanged:: 0.5
             - removed ``keep_temp`` keyword argument, which was only relevant
               for MongoDB server versions older than 1.7.4
  
          .. versionadded:: 0.3
          """
          queryset = self.clone()
  
          MapReduceDocument = _import_class("MapReduceDocument")
  
          if not hasattr(self._collection, "map_reduce"):
              raise NotImplementedError("Requires MongoDB >= 1.7.1")
  
          map_f_scope = {}
          if isinstance(map_f, Code):
              map_f_scope = map_f.scope
              map_f = str(map_f)
          map_f = Code(queryset._sub_js_fields(map_f), map_f_scope)
  
          reduce_f_scope = {}
          if isinstance(reduce_f, Code):
              reduce_f_scope = reduce_f.scope
              reduce_f = str(reduce_f)
          reduce_f_code = queryset._sub_js_fields(reduce_f)
          reduce_f = Code(reduce_f_code, reduce_f_scope)
  
          mr_args = {"query": queryset._query}
  
          if finalize_f:
              finalize_f_scope = {}
              if isinstance(finalize_f, Code):
                  finalize_f_scope = finalize_f.scope
                  finalize_f = str(finalize_f)
              finalize_f_code = queryset._sub_js_fields(finalize_f)
              finalize_f = Code(finalize_f_code, finalize_f_scope)
              mr_args["finalize"] = finalize_f
  
          if scope:
              mr_args["scope"] = scope
  
          if limit:
              mr_args["limit"] = limit
  
          if output == "inline" and not queryset._ordering:
              map_reduce_function = "inline_map_reduce"
          else:
              map_reduce_function = "map_reduce"
  
              if isinstance(output, str):
                  mr_args["out"] = output
  
              elif isinstance(output, dict):
                  ordered_output = []
  
                  for part in ("replace", "merge", "reduce"):
                      value = output.get(part)
                      if value:
                          ordered_output.append((part, value))
                          break
  
                  else:
                      raise OperationError("actionData not specified for output")
  
                  db_alias = output.get("db_alias")
                  remaing_args = ["db", "sharded", "nonAtomic"]
  
                  if db_alias:
                      ordered_output.append(("db", get_db(db_alias).name))
                      del remaing_args[0]
  
                  for part in remaing_args:
                      value = output.get(part)
                      if value:
                          ordered_output.append((part, value))
  
                  mr_args["out"] = SON(ordered_output)
  
          results = getattr(queryset._collection, map_reduce_function)(
              map_f, reduce_f, **mr_args
          )
  
          if map_reduce_function == "map_reduce":
              results = results.find()
  
          if queryset._ordering:
              results = results.sort(queryset._ordering)
  
          for doc in results:
              yield MapReduceDocument(
                  queryset._document, queryset._collection, doc["_id"], doc["value"]
              )
  
      def where(self, where_clause):
          """Filter ``QuerySet`` results with a ``$where`` clause (a Javascript
          expression). Performs automatic field name substitution like
          :meth:`mongoengine.queryset.Queryset.exec_js`.
  
          .. note:: When using this mode of query, the database will call your
                    function, or evaluate your predicate clause, for each object
                    in the collection.
  
          .. versionadded:: 0.5
          """
          queryset = self.clone()
          where_clause = queryset._sub_js_fields(where_clause)
          queryset._where_clause = where_clause
          return queryset
  
      def sum(self, field):
          """Sum over the values of the specified field.
  
          :param field: the field to sum over; use dot notation to refer to
              embedded document fields
          """
          db_field = self._fields_to_dbfields([field]).pop()
          pipeline = [
              {"$match": self._query},
              {"$group": {"_id": "sum", "total": {"$sum": "$" + db_field}}},
          ]
  
          # if we're performing a sum over a list field, we sum up all the
          # elements in the list, hence we need to $unwind the arrays first
          ListField = _import_class("ListField")
          field_parts = field.split(".")
          field_instances = self._document._lookup_field(field_parts)
          if isinstance(field_instances[-1], ListField):
              pipeline.insert(1, {"$unwind": "$" + field})
  
          result = tuple(self._document._get_collection().aggregate(pipeline))
  
          if result:
              return result[0]["total"]
          return 0
  
      def average(self, field):
          """Average over the values of the specified field.
  
          :param field: the field to average over; use dot notation to refer to
              embedded document fields
          """
          db_field = self._fields_to_dbfields([field]).pop()
          pipeline = [
              {"$match": self._query},
              {"$group": {"_id": "avg", "total": {"$avg": "$" + db_field}}},
          ]
  
          # if we're performing an average over a list field, we average out
          # all the elements in the list, hence we need to $unwind the arrays
          # first
          ListField = _import_class("ListField")
          field_parts = field.split(".")
          field_instances = self._document._lookup_field(field_parts)
          if isinstance(field_instances[-1], ListField):
              pipeline.insert(1, {"$unwind": "$" + field})
  
          result = tuple(self._document._get_collection().aggregate(pipeline))
          if result:
              return result[0]["total"]
          return 0
  
      # Iterator helpers
  
      def __next__(self):
          """Wrap the result in a :class:`~mongoengine.Document` object.
          """
          if self._limit == 0 or self._none:
              raise StopIteration
  
          raw_doc = next(self._cursor)
  
          if self._as_pymongo:
              return raw_doc
  
          doc = self._document._from_son(
              raw_doc,
              _auto_dereference=self._auto_dereference,
              only_fields=self.only_fields,
          )
  
          if self._scalar:
              return self._get_scalar(doc)
  
          return doc
  
      next = __next__  # For Python2 support
  
      def rewind(self):
          """Rewind the cursor to its unevaluated state.
  
          .. versionadded:: 0.3
          """
          self._iter = False
          self._cursor.rewind()
  
      # Properties
  
      @property
      def _collection(self):
          """Property that returns the collection object. This allows us to
          perform operations only if the collection is accessed.
          """
          return self._collection_obj
  
      @property
      def _cursor_args(self):
          fields_name = "projection"
          cursor_args = {}
          if not self._timeout:
              cursor_args["no_cursor_timeout"] = True
  
          if self._loaded_fields:
              cursor_args[fields_name] = self._loaded_fields.as_dict()
  
          if self._search_text:
              if fields_name not in cursor_args:
                  cursor_args[fields_name] = {}
  
              cursor_args[fields_name]["_text_score"] = {"$meta": "textScore"}
  
          return cursor_args
  
      @property
      def _cursor(self):
          """Return a PyMongo cursor object corresponding to this queryset."""
  
          # If _cursor_obj already exists, return it immediately.
          if self._cursor_obj is not None:
              return self._cursor_obj
  
          # Create a new PyMongo cursor.
          # XXX In PyMongo 3+, we define the read preference on a collection
          # level, not a cursor level. Thus, we need to get a cloned collection
          # object using `with_options` first.
          if self._read_preference is not None:
              self._cursor_obj = self._collection.with_options(
                  read_preference=self._read_preference
              ).find(self._query, **self._cursor_args)
          else:
              self._cursor_obj = self._collection.find(self._query, **self._cursor_args)
  
          # Apply "where" clauses to cursor
          if self._where_clause:
              where_clause = self._sub_js_fields(self._where_clause)
              self._cursor_obj.where(where_clause)
  
          # Apply ordering to the cursor.
          # XXX self._ordering can be equal to:
          # * None if we didn't explicitly call order_by on this queryset.
          # * A list of PyMongo-style sorting tuples.
          # * An empty list if we explicitly called order_by() without any
          #   arguments. This indicates that we want to clear the default
          #   ordering.
          if self._ordering:
              # explicit ordering
              self._cursor_obj.sort(self._ordering)
          elif self._ordering is None and self._document._meta["ordering"]:
              # default ordering
              order = self._get_order_by(self._document._meta["ordering"])
              self._cursor_obj.sort(order)
  
          if self._limit is not None:
              self._cursor_obj.limit(self._limit)
  
          if self._skip is not None:
              self._cursor_obj.skip(self._skip)
  
          if self._hint != -1:
              self._cursor_obj.hint(self._hint)
  
          if self._collation is not None:
              self._cursor_obj.collation(self._collation)
  
          if self._batch_size is not None:
              self._cursor_obj.batch_size(self._batch_size)
  
          if self._comment is not None:
              self._cursor_obj.comment(self._comment)
  
          return self._cursor_obj
  
      def __deepcopy__(self, memo):
          """Essential for chained queries with ReferenceFields involved"""
          return self.clone()
  
      @property
      def _query(self):
          if self._mongo_query is None:
              self._mongo_query = self._query_obj.to_query(self._document)
              if self._cls_query:
                  if "_cls" in self._mongo_query:
                      self._mongo_query = {"$and": [self._cls_query, self._mongo_query]}
                  else:
                      self._mongo_query.update(self._cls_query)
          return self._mongo_query
  
      @property
      def _dereference(self):
          if not self.__dereference:
              self.__dereference = _import_class("DeReference")()
          return self.__dereference
  
      def no_dereference(self):
          """Turn off any dereferencing for the results of this queryset."""
          queryset = self.clone()
          queryset._auto_dereference = False
          return queryset
  
      # Helper Functions
  
      def _fields_to_dbfields(self, fields):
          """Translate fields' paths to their db equivalents."""
          subclasses = []
          if self._document._meta["allow_inheritance"]:
              subclasses = [get_document(x) for x in self._document._subclasses][1:]
  
          db_field_paths = []
          for field in fields:
              field_parts = field.split(".")
              try:
                  field = ".".join(
                      f if isinstance(f, str) else f.db_field
                      for f in self._document._lookup_field(field_parts)
                  )
                  db_field_paths.append(field)
              except LookUpError as err:
                  found = False
  
                  # If a field path wasn't found on the main document, go
                  # through its subclasses and see if it exists on any of them.
                  for subdoc in subclasses:
                      try:
                          subfield = ".".join(
                              f if isinstance(f, str) else f.db_field
                              for f in subdoc._lookup_field(field_parts)
                          )
                          db_field_paths.append(subfield)
                          found = True
                          break
                      except LookUpError:
                          pass
  
                  if not found:
                      raise err
  
          return db_field_paths
  
      def _get_order_by(self, keys):
          """Given a list of MongoEngine-style sort keys, return a list
          of sorting tuples that can be applied to a PyMongo cursor. For
          example:
  
          >>> qs._get_order_by(['-last_name', 'first_name'])
          [('last_name', -1), ('first_name', 1)]
          """
          key_list = []
          for key in keys:
              if not key:
                  continue
  
              if key == "$text_score":
                  key_list.append(("_text_score", {"$meta": "textScore"}))
                  continue
  
              direction = pymongo.ASCENDING
              if key[0] == "-":
                  direction = pymongo.DESCENDING
  
              if key[0] in ("-", "+"):
                  key = key[1:]
  
              key = key.replace("__", ".")
              try:
                  key = self._document._translate_field_name(key)
              except Exception:
                  # TODO this exception should be more specific
                  pass
  
              key_list.append((key, direction))
  
          return key_list
  
      def _get_scalar(self, doc):
          def lookup(obj, name):
              chunks = name.split("__")
              for chunk in chunks:
                  obj = getattr(obj, chunk)
              return obj
  
          data = [lookup(doc, n) for n in self._scalar]
          if len(data) == 1:
              return data[0]
  
          return tuple(data)
  
      def _sub_js_fields(self, code):
          """When fields are specified with [~fieldname] syntax, where
          *fieldname* is the Python name of a field, *fieldname* will be
          substituted for the MongoDB name of the field (specified using the
          :attr:`name` keyword argument in a field's constructor).
          """
  
          def field_sub(match):
              # Extract just the field name, and look up the field objects
              field_name = match.group(1).split(".")
              fields = self._document._lookup_field(field_name)
              # Substitute the correct name for the field into the javascript
              return '["%s"]' % fields[-1].db_field
  
          def field_path_sub(match):
              # Extract just the field name, and look up the field objects
              field_name = match.group(1).split(".")
              fields = self._document._lookup_field(field_name)
              # Substitute the correct name for the field into the javascript
              return ".".join([f.db_field for f in fields])
  
          code = re.sub(r"\[\s*~([A-z_][A-z_0-9.]+?)\s*\]", field_sub, code)
          code = re.sub(r"\{\{\s*~([A-z_][A-z_0-9.]+?)\s*\}\}", field_path_sub, code)
          return code
  
      def _chainable_method(self, method_name, val):
          """Call a particular method on the PyMongo cursor call a particular chainable method
          with the provided value.
          """
          queryset = self.clone()
  
          # Get an existing cursor object or create a new one
          cursor = queryset._cursor
  
          # Find the requested method on the cursor and call it with the
          # provided value
          getattr(cursor, method_name)(val)
  
          # Cache the value on the queryset._{method_name}
          setattr(queryset, "_" + method_name, val)
  
          return queryset
  