manyi/venv/Lib/site-packages/mongoengine/document.py

import re
import warnings

from bson.dbref import DBRef
import pymongo
from pymongo.read_preferences import ReadPreference
import six

from mongoengine import signals
from mongoengine.base import (BaseDict, BaseDocument, BaseList,
                              DocumentMetaclass, EmbeddedDocumentList,
                              TopLevelDocumentMetaclass, get_document)
from mongoengine.common import _import_class
from mongoengine.connection import DEFAULT_CONNECTION_NAME, get_db
from mongoengine.context_managers import switch_collection, switch_db
from mongoengine.errors import (InvalidDocumentError, InvalidQueryError,
                                SaveConditionError)
from mongoengine.python_support import IS_PYMONGO_3
from mongoengine.queryset import (NotUniqueError, OperationError,
                                  QuerySet, transform)

__all__ = ('Document', 'EmbeddedDocument', 'DynamicDocument',
           'DynamicEmbeddedDocument', 'OperationError',
           'InvalidCollectionError', 'NotUniqueError', 'MapReduceDocument')


def includes_cls(fields):
    """Helper function used for ensuring and comparing indexes."""
    first_field = None
    if len(fields):
        if isinstance(fields[0], six.string_types):
            first_field = fields[0]
        elif isinstance(fields[0], (list, tuple)) and len(fields[0]):
            first_field = fields[0][0]
    return first_field == '_cls'


class InvalidCollectionError(Exception):
    pass


class EmbeddedDocument(six.with_metaclass(DocumentMetaclass, BaseDocument)):
    """A :class:`~mongoengine.Document` that isn't stored in its own
    collection.  :class:`~mongoengine.EmbeddedDocument`\ s should be used as
    fields on :class:`~mongoengine.Document`\ s through the
    :class:`~mongoengine.EmbeddedDocumentField` field type.

    A :class:`~mongoengine.EmbeddedDocument` subclass may be itself subclassed,
    to create a specialised version of the embedded document that will be
    stored in the same collection. To facilitate this behaviour a `_cls`
    field is added to documents (hidden though the MongoEngine interface).
    To enable this behaviour set :attr:`allow_inheritance` to ``True`` in the
    :attr:`meta` dictionary.
    """

    __slots__ = ('_instance', )

    # The __metaclass__ attribute is removed by 2to3 when running with Python3
    # my_metaclass is defined so that metaclass can be queried in Python 2 & 3
    my_metaclass = DocumentMetaclass

    # A generic embedded document doesn't have any immutable properties
    # that describe it uniquely, hence it shouldn't be hashable. You can
    # define your own __hash__ method on a subclass if you need your
    # embedded documents to be hashable.
    __hash__ = None

    def __init__(self, *args, **kwargs):
        super(EmbeddedDocument, self).__init__(*args, **kwargs)
        self._instance = None
        self._changed_fields = []

    def __eq__(self, other):
        if isinstance(other, self.__class__):
            return self._data == other._data
        return False

    def __ne__(self, other):
        return not self.__eq__(other)

    def to_mongo(self, *args, **kwargs):
        data = super(EmbeddedDocument, self).to_mongo(*args, **kwargs)

        # remove _id from the SON if it's in it and it's None
        if '_id' in data and data['_id'] is None:
            del data['_id']

        return data

    def save(self, *args, **kwargs):
        self._instance.save(*args, **kwargs)

    def reload(self, *args, **kwargs):
        self._instance.reload(*args, **kwargs)


class Document(six.with_metaclass(TopLevelDocumentMetaclass, BaseDocument)):
    """The base class used for defining the structure and properties of
    collections of documents stored in MongoDB. Inherit from this class, and
    add fields as class attributes to define a document's structure.
    Individual documents may then be created by making instances of the
    :class:`~mongoengine.Document` subclass.

    By default, the MongoDB collection used to store documents created using a
    :class:`~mongoengine.Document` subclass will be the name of the subclass
    converted to lowercase. A different collection may be specified by
    providing :attr:`collection` to the :attr:`meta` dictionary in the class
    definition.

    A :class:`~mongoengine.Document` subclass may be itself subclassed, to
    create a specialised version of the document that will be stored in the
    same collection. To facilitate this behaviour a `_cls`
    field is added to documents (hidden though the MongoEngine interface).
    To enable this behaviourset :attr:`allow_inheritance` to ``True`` in the
    :attr:`meta` dictionary.

    A :class:`~mongoengine.Document` may use a **Capped Collection** by
    specifying :attr:`max_documents` and :attr:`max_size` in the :attr:`meta`
    dictionary. :attr:`max_documents` is the maximum number of documents that
    is allowed to be stored in the collection, and :attr:`max_size` is the
    maximum size of the collection in bytes. :attr:`max_size` is rounded up
    to the next multiple of 256 by MongoDB internally and mongoengine before.
    Use also a multiple of 256 to avoid confusions.  If :attr:`max_size` is not
    specified and :attr:`max_documents` is, :attr:`max_size` defaults to
    10485760 bytes (10MB).

    Indexes may be created by specifying :attr:`indexes` in the :attr:`meta`
    dictionary. The value should be a list of field names or tuples of field
    names. Index direction may be specified by prefixing the field names with
    a **+** or **-** sign.

    Automatic index creation can be disabled by specifying
    :attr:`auto_create_index` in the :attr:`meta` dictionary. If this is set to
    False then indexes will not be created by MongoEngine.  This is useful in
    production systems where index creation is performed as part of a
    deployment system.

    By default, _cls will be added to the start of every index (that
    doesn't contain a list) if allow_inheritance is True. This can be
    disabled by either setting cls to False on the specific index or
    by setting index_cls to False on the meta dictionary for the document.

    By default, any extra attribute existing in stored data but not declared
    in your model will raise a :class:`~mongoengine.FieldDoesNotExist` error.
    This can be disabled by setting :attr:`strict` to ``False``
    in the :attr:`meta` dictionary.
    """

    # The __metaclass__ attribute is removed by 2to3 when running with Python3
    # my_metaclass is defined so that metaclass can be queried in Python 2 & 3
    my_metaclass = TopLevelDocumentMetaclass

    __slots__ = ('__objects',)

    @property
    def pk(self):
        """Get the primary key."""
        if 'id_field' not in self._meta:
            return None
        return getattr(self, self._meta['id_field'])

    @pk.setter
    def pk(self, value):
        """Set the primary key."""
        return setattr(self, self._meta['id_field'], value)

    def __hash__(self):
        """Return the hash based on the PK of this document. If it's new
        and doesn't have a PK yet, return the default object hash instead.
        """
        if self.pk is None:
            return super(BaseDocument, self).__hash__()

        return hash(self.pk)

    @classmethod
    def _get_db(cls):
        """Some Model using other db_alias"""
        return get_db(cls._meta.get('db_alias', DEFAULT_CONNECTION_NAME))

    @classmethod
    def _get_collection(cls):
        """Return a PyMongo collection for the document."""
        if not hasattr(cls, '_collection') or cls._collection is None:

            # Get the collection, either capped or regular.
            if cls._meta.get('max_size') or cls._meta.get('max_documents'):
                cls._collection = cls._get_capped_collection()
            else:
                db = cls._get_db()
                collection_name = cls._get_collection_name()
                cls._collection = db[collection_name]

            # Ensure indexes on the collection unless auto_create_index was
            # set to False.
            # Also there is no need to ensure indexes on slave.
            db = cls._get_db()
            if cls._meta.get('auto_create_index', True) and\
                    db.client.is_primary:
                cls.ensure_indexes()

        return cls._collection

    @classmethod
    def _get_capped_collection(cls):
        """Create a new or get an existing capped PyMongo collection."""
        db = cls._get_db()
        collection_name = cls._get_collection_name()

        # Get max document limit and max byte size from meta.
        max_size = cls._meta.get('max_size') or 10 * 2 ** 20  # 10MB default
        max_documents = cls._meta.get('max_documents')

        # MongoDB will automatically raise the size to make it a multiple of
        # 256 bytes. We raise it here ourselves to be able to reliably compare
        # the options below.
        if max_size % 256:
            max_size = (max_size // 256 + 1) * 256

        # If the collection already exists and has different options
        # (i.e. isn't capped or has different max/size), raise an error.
        if collection_name in db.collection_names():
            collection = db[collection_name]
            options = collection.options()
            if (
                options.get('max') != max_documents or
                options.get('size') != max_size
            ):
                raise InvalidCollectionError(
                    'Cannot create collection "{}" as a capped '
                    'collection as it already exists'.format(cls._collection)
                )

            return collection

        # Create a new capped collection.
        opts = {'capped': True, 'size': max_size}
        if max_documents:
            opts['max'] = max_documents

        return db.create_collection(collection_name, **opts)

    def to_mongo(self, *args, **kwargs):
        data = super(Document, self).to_mongo(*args, **kwargs)

        # If '_id' is None, try and set it from self._data. If that
        # doesn't exist either, remote '_id' from the SON completely.
        if data['_id'] is None:
            if self._data.get('id') is None:
                del data['_id']
            else:
                data['_id'] = self._data['id']

        return data

    def modify(self, query=None, **update):
        """Perform an atomic update of the document in the database and reload
        the document object using updated version.

        Returns True if the document has been updated or False if the document
        in the database doesn't match the query.

        .. note:: All unsaved changes that have been made to the document are
            rejected if the method returns True.

        :param query: the update will be performed only if the document in the
            database matches the query
        :param update: Django-style update keyword arguments
        """
        if query is None:
            query = {}

        if self.pk is None:
            raise InvalidDocumentError('The document does not have a primary key.')

        id_field = self._meta['id_field']
        query = query.copy() if isinstance(query, dict) else query.to_query(self)

        if id_field not in query:
            query[id_field] = self.pk
        elif query[id_field] != self.pk:
            raise InvalidQueryError('Invalid document modify query: it must modify only this document.')

        # Need to add shard key to query, or you get an error
        query.update(self._object_key)

        updated = self._qs(**query).modify(new=True, **update)
        if updated is None:
            return False

        for field in self._fields_ordered:
            setattr(self, field, self._reload(field, updated[field]))

        self._changed_fields = updated._changed_fields
        self._created = False

        return True

    def save(self, force_insert=False, validate=True, clean=True,
             write_concern=None, cascade=None, cascade_kwargs=None,
             _refs=None, save_condition=None, signal_kwargs=None, **kwargs):
        """Save the :class:`~mongoengine.Document` to the database. If the
        document already exists, it will be updated, otherwise it will be
        created.

        :param force_insert: only try to create a new document, don't allow
            updates of existing documents.
        :param validate: validates the document; set to ``False`` to skip.
        :param clean: call the document clean method, requires `validate` to be
            True.
        :param write_concern: Extra keyword arguments are passed down to
            :meth:`~pymongo.collection.Collection.save` OR
            :meth:`~pymongo.collection.Collection.insert`
            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 cascade: Sets the flag for cascading saves.  You can set a
            default by setting "cascade" in the document __meta__
        :param cascade_kwargs: (optional) kwargs dictionary to be passed throw
            to cascading saves.  Implies ``cascade=True``.
        :param _refs: A list of processed references used in cascading saves
        :param save_condition: only perform save if matching record in db
            satisfies condition(s) (e.g. version number).
            Raises :class:`OperationError` if the conditions are not satisfied
        :param signal_kwargs: (optional) kwargs dictionary to be passed to
            the signal calls.

        .. versionchanged:: 0.5
            In existing documents it only saves changed fields using
            set / unset.  Saves are cascaded and any
            :class:`~bson.dbref.DBRef` objects that have changes are
            saved as well.
        .. versionchanged:: 0.6
            Added cascading saves
        .. versionchanged:: 0.8
            Cascade saves are optional and default to False.  If you want
            fine grain control then you can turn off using document
            meta['cascade'] = True.  Also you can pass different kwargs to
            the cascade save using cascade_kwargs which overwrites the
            existing kwargs with custom values.
        .. versionchanged:: 0.8.5
            Optional save_condition that only overwrites existing documents
            if the condition is satisfied in the current db record.
        .. versionchanged:: 0.10
            :class:`OperationError` exception raised if save_condition fails.
        .. versionchanged:: 0.10.1
            :class: save_condition failure now raises a `SaveConditionError`
        .. versionchanged:: 0.10.7
            Add signal_kwargs argument
        """
        if self._meta.get('abstract'):
            raise InvalidDocumentError('Cannot save an abstract document.')

        signal_kwargs = signal_kwargs or {}
        signals.pre_save.send(self.__class__, document=self, **signal_kwargs)

        if validate:
            self.validate(clean=clean)

        if write_concern is None:
            write_concern = {'w': 1}

        doc = self.to_mongo()

        created = ('_id' not in doc or self._created or force_insert)

        signals.pre_save_post_validation.send(self.__class__, document=self,
                                              created=created, **signal_kwargs)
        # it might be refreshed by the pre_save_post_validation hook, e.g., for etag generation
        doc = self.to_mongo()

        if self._meta.get('auto_create_index', True):
            self.ensure_indexes()

        try:
            # Save a new document or update an existing one
            if created:
                object_id = self._save_create(doc, force_insert, write_concern)
            else:
                object_id, created = self._save_update(doc, save_condition,
                                                       write_concern)

            if cascade is None:
                cascade = (self._meta.get('cascade', False) or
                           cascade_kwargs is not None)

            if cascade:
                kwargs = {
                    'force_insert': force_insert,
                    'validate': validate,
                    'write_concern': write_concern,
                    'cascade': cascade
                }
                if cascade_kwargs:  # Allow granular control over cascades
                    kwargs.update(cascade_kwargs)
                kwargs['_refs'] = _refs
                self.cascade_save(**kwargs)

        except pymongo.errors.DuplicateKeyError as err:
            message = u'Tried to save duplicate unique keys (%s)'
            raise NotUniqueError(message % six.text_type(err))
        except pymongo.errors.OperationFailure as err:
            message = 'Could not save document (%s)'
            if re.match('^E1100[01] duplicate key', six.text_type(err)):
                # E11000 - duplicate key error index
                # E11001 - duplicate key on update
                message = u'Tried to save duplicate unique keys (%s)'
                raise NotUniqueError(message % six.text_type(err))
            raise OperationError(message % six.text_type(err))

        # Make sure we store the PK on this document now that it's saved
        id_field = self._meta['id_field']
        if created or id_field not in self._meta.get('shard_key', []):
            self[id_field] = self._fields[id_field].to_python(object_id)

        signals.post_save.send(self.__class__, document=self,
                               created=created, **signal_kwargs)

        self._clear_changed_fields()
        self._created = False

        return self

    def _save_create(self, doc, force_insert, write_concern):
        """Save a new document.

        Helper method, should only be used inside save().
        """
        collection = self._get_collection()

        if force_insert:
            return collection.insert(doc, **write_concern)

        object_id = collection.save(doc, **write_concern)

        # In PyMongo 3.0, the save() call calls internally the _update() call
        # but they forget to return the _id value passed back, therefore getting it back here
        # Correct behaviour in 2.X and in 3.0.1+ versions
        if not object_id and pymongo.version_tuple == (3, 0):
            pk_as_mongo_obj = self._fields.get(self._meta['id_field']).to_mongo(self.pk)
            object_id = (
                self._qs.filter(pk=pk_as_mongo_obj).first() and
                self._qs.filter(pk=pk_as_mongo_obj).first().pk
            )  # TODO doesn't this make 2 queries?

        return object_id

    def _get_update_doc(self):
        """Return a dict containing all the $set and $unset operations
        that should be sent to MongoDB based on the changes made to this
        Document.
        """
        updates, removals = self._delta()

        update_doc = {}
        if updates:
            update_doc['$set'] = updates
        if removals:
            update_doc['$unset'] = removals

        return update_doc

    def _save_update(self, doc, save_condition, write_concern):
        """Update an existing document.

        Helper method, should only be used inside save().
        """
        collection = self._get_collection()
        object_id = doc['_id']
        created = False

        select_dict = {}
        if save_condition is not None:
            select_dict = transform.query(self.__class__, **save_condition)

        select_dict['_id'] = object_id

        # Need to add shard key to query, or you get an error
        shard_key = self._meta.get('shard_key', tuple())
        for k in shard_key:
            path = self._lookup_field(k.split('.'))
            actual_key = [p.db_field for p in path]
            val = doc
            for ak in actual_key:
                val = val[ak]
            select_dict['.'.join(actual_key)] = val

        update_doc = self._get_update_doc()
        if update_doc:
            upsert = save_condition is None
            last_error = collection.update(select_dict, update_doc,
                                           upsert=upsert, **write_concern)
            if not upsert and last_error['n'] == 0:
                raise SaveConditionError('Race condition preventing'
                                         ' document update detected')
            if last_error is not None:
                updated_existing = last_error.get('updatedExisting')
                if updated_existing is False:
                    created = True
                    # !!! This is bad, means we accidentally created a new,
                    # potentially corrupted document. See
                    # https://github.com/MongoEngine/mongoengine/issues/564

        return object_id, created

    def cascade_save(self, **kwargs):
        """Recursively save any references and generic references on the
        document.
        """
        _refs = kwargs.get('_refs') or []

        ReferenceField = _import_class('ReferenceField')
        GenericReferenceField = _import_class('GenericReferenceField')

        for name, cls in self._fields.items():
            if not isinstance(cls, (ReferenceField,
                                    GenericReferenceField)):
                continue

            ref = self._data.get(name)
            if not ref or isinstance(ref, DBRef):
                continue

            if not getattr(ref, '_changed_fields', True):
                continue

            ref_id = "%s,%s" % (ref.__class__.__name__, str(ref._data))
            if ref and ref_id not in _refs:
                _refs.append(ref_id)
                kwargs["_refs"] = _refs
                ref.save(**kwargs)
                ref._changed_fields = []

    @property
    def _qs(self):
        """Return the queryset to use for updating / reloading / deletions."""
        if not hasattr(self, '__objects'):
            self.__objects = QuerySet(self, self._get_collection())
        return self.__objects

    @property
    def _object_key(self):
        """Get the query dict that can be used to fetch this object from
        the database. Most of the time it's a simple PK lookup, but in
        case of a sharded collection with a compound shard key, it can
        contain a more complex query.
        """
        select_dict = {'pk': self.pk}
        shard_key = self.__class__._meta.get('shard_key', tuple())
        for k in shard_key:
            path = self._lookup_field(k.split('.'))
            actual_key = [p.db_field for p in path]
            val = self
            for ak in actual_key:
                val = getattr(val, ak)
            select_dict['__'.join(actual_key)] = val
        return select_dict

    def update(self, **kwargs):
        """Performs an update on the :class:`~mongoengine.Document`
        A convenience wrapper to :meth:`~mongoengine.QuerySet.update`.

        Raises :class:`OperationError` if called on an object that has not yet
        been saved.
        """
        if self.pk is None:
            if kwargs.get('upsert', False):
                query = self.to_mongo()
                if '_cls' in query:
                    del query['_cls']
                return self._qs.filter(**query).update_one(**kwargs)
            else:
                raise OperationError(
                    'attempt to update a document not yet saved')

        # Need to add shard key to query, or you get an error
        return self._qs.filter(**self._object_key).update_one(**kwargs)

    def delete(self, signal_kwargs=None, **write_concern):
        """Delete the :class:`~mongoengine.Document` from the database. This
        will only take effect if the document has been previously saved.

        :param signal_kwargs: (optional) kwargs dictionary to be passed to
            the signal calls.
        :param write_concern: Extra keyword arguments are passed down which
            will be used as options for the resultant ``getLastError`` command.
            For example, ``save(..., w: 2, fsync: True)`` will
            wait until at least two servers have recorded the write and
            will force an fsync on the primary server.

        .. versionchanged:: 0.10.7
            Add signal_kwargs argument
        """
        signal_kwargs = signal_kwargs or {}
        signals.pre_delete.send(self.__class__, document=self, **signal_kwargs)

        # Delete FileFields separately
        FileField = _import_class('FileField')
        for name, field in self._fields.iteritems():
            if isinstance(field, FileField):
                getattr(self, name).delete()

        try:
            self._qs.filter(
                **self._object_key).delete(write_concern=write_concern, _from_doc_delete=True)
        except pymongo.errors.OperationFailure as err:
            message = u'Could not delete document (%s)' % err.message
            raise OperationError(message)
        signals.post_delete.send(self.__class__, document=self, **signal_kwargs)

    def switch_db(self, db_alias, keep_created=True):
        """
        Temporarily switch the database for a document instance.

        Only really useful for archiving off data and calling `save()`::

            user = User.objects.get(id=user_id)
            user.switch_db('archive-db')
            user.save()

        :param str db_alias: The database alias to use for saving the document

        :param bool keep_created: keep self._created value after switching db, else is reset to True


        .. seealso::
            Use :class:`~mongoengine.context_managers.switch_collection`
            if you need to read from another collection
        """
        with switch_db(self.__class__, db_alias) as cls:
            collection = cls._get_collection()
            db = cls._get_db()
        self._get_collection = lambda: collection
        self._get_db = lambda: db
        self._collection = collection
        self._created = True if not keep_created else self._created
        self.__objects = self._qs
        self.__objects._collection_obj = collection
        return self

    def switch_collection(self, collection_name, keep_created=True):
        """
        Temporarily switch the collection for a document instance.

        Only really useful for archiving off data and calling `save()`::

            user = User.objects.get(id=user_id)
            user.switch_collection('old-users')
            user.save()

        :param str collection_name: The database alias to use for saving the
            document

        :param bool keep_created: keep self._created value after switching collection, else is reset to True


        .. seealso::
            Use :class:`~mongoengine.context_managers.switch_db`
            if you need to read from another database
        """
        with switch_collection(self.__class__, collection_name) as cls:
            collection = cls._get_collection()
        self._get_collection = lambda: collection
        self._collection = collection
        self._created = True if not keep_created else self._created
        self.__objects = self._qs
        self.__objects._collection_obj = collection
        return self

    def select_related(self, max_depth=1):
        """Handles dereferencing of :class:`~bson.dbref.DBRef` objects to
        a maximum depth in order to cut down the number queries to mongodb.

        .. versionadded:: 0.5
        """
        DeReference = _import_class('DeReference')
        DeReference()([self], max_depth + 1)
        return self

    def reload(self, *fields, **kwargs):
        """Reloads all attributes from the database.

        :param fields: (optional) args list of fields to reload
        :param max_depth: (optional) depth of dereferencing to follow

        .. versionadded:: 0.1.2
        .. versionchanged:: 0.6  Now chainable
        .. versionchanged:: 0.9  Can provide specific fields to reload
        """
        max_depth = 1
        if fields and isinstance(fields[0], int):
            max_depth = fields[0]
            fields = fields[1:]
        elif 'max_depth' in kwargs:
            max_depth = kwargs['max_depth']

        if self.pk is None:
            raise self.DoesNotExist('Document does not exist')

        obj = self._qs.read_preference(ReadPreference.PRIMARY).filter(
            **self._object_key).only(*fields).limit(
            1).select_related(max_depth=max_depth)

        if obj:
            obj = obj[0]
        else:
            raise self.DoesNotExist('Document does not exist')
        for field in obj._data:
            if not fields or field in fields:
                try:
                    setattr(self, field, self._reload(field, obj[field]))
                except (KeyError, AttributeError):
                    try:
                        # If field is a special field, e.g. items is stored as _reserved_items,
                        # a KeyError is thrown. So try to retrieve the field from _data
                        setattr(self, field, self._reload(field, obj._data.get(field)))
                    except KeyError:
                        # If field is removed from the database while the object
                        # is in memory, a reload would cause a KeyError
                        # i.e. obj.update(unset__field=1) followed by obj.reload()
                        delattr(self, field)

        self._changed_fields = list(
            set(self._changed_fields) - set(fields)
        ) if fields else obj._changed_fields
        self._created = False
        return self

    def _reload(self, key, value):
        """Used by :meth:`~mongoengine.Document.reload` to ensure the
        correct instance is linked to self.
        """
        if isinstance(value, BaseDict):
            value = [(k, self._reload(k, v)) for k, v in value.items()]
            value = BaseDict(value, self, key)
        elif isinstance(value, EmbeddedDocumentList):
            value = [self._reload(key, v) for v in value]
            value = EmbeddedDocumentList(value, self, key)
        elif isinstance(value, BaseList):
            value = [self._reload(key, v) for v in value]
            value = BaseList(value, self, key)
        elif isinstance(value, (EmbeddedDocument, DynamicEmbeddedDocument)):
            value._instance = None
            value._changed_fields = []
        return value

    def to_dbref(self):
        """Returns an instance of :class:`~bson.dbref.DBRef` useful in
        `__raw__` queries."""
        if self.pk is None:
            msg = 'Only saved documents can have a valid dbref'
            raise OperationError(msg)
        return DBRef(self.__class__._get_collection_name(), self.pk)

    @classmethod
    def register_delete_rule(cls, document_cls, field_name, rule):
        """This method registers the delete rules to apply when removing this
        object.
        """
        classes = [get_document(class_name)
                   for class_name in cls._subclasses
                   if class_name != cls.__name__] + [cls]
        documents = [get_document(class_name)
                     for class_name in document_cls._subclasses
                     if class_name != document_cls.__name__] + [document_cls]

        for klass in classes:
            for document_cls in documents:
                delete_rules = klass._meta.get('delete_rules') or {}
                delete_rules[(document_cls, field_name)] = rule
                klass._meta['delete_rules'] = delete_rules

    @classmethod
    def drop_collection(cls):
        """Drops the entire collection associated with this
        :class:`~mongoengine.Document` type from the database.

        Raises :class:`OperationError` if the document has no collection set
        (i.g. if it is `abstract`)

        .. versionchanged:: 0.10.7
            :class:`OperationError` exception raised if no collection available
        """
        col_name = cls._get_collection_name()
        if not col_name:
            raise OperationError('Document %s has no collection defined '
                                 '(is it abstract ?)' % cls)
        cls._collection = None
        db = cls._get_db()
        db.drop_collection(col_name)

    @classmethod
    def create_index(cls, keys, background=False, **kwargs):
        """Creates the given indexes if required.

        :param keys: a single index key or a list of index keys (to
            construct a multi-field index); keys may be prefixed with a **+**
            or a **-** to determine the index ordering
        :param background: Allows index creation in the background
        """
        index_spec = cls._build_index_spec(keys)
        index_spec = index_spec.copy()
        fields = index_spec.pop('fields')
        drop_dups = kwargs.get('drop_dups', False)
        if IS_PYMONGO_3 and drop_dups:
            msg = 'drop_dups is deprecated and is removed when using PyMongo 3+.'
            warnings.warn(msg, DeprecationWarning)
        elif not IS_PYMONGO_3:
            index_spec['drop_dups'] = drop_dups
        index_spec['background'] = background
        index_spec.update(kwargs)

        if IS_PYMONGO_3:
            return cls._get_collection().create_index(fields, **index_spec)
        else:
            return cls._get_collection().ensure_index(fields, **index_spec)

    @classmethod
    def ensure_index(cls, key_or_list, drop_dups=False, background=False,
                     **kwargs):
        """Ensure that the given indexes are in place. Deprecated in favour
        of create_index.

        :param key_or_list: a single index key or a list of index keys (to
            construct a multi-field index); keys may be prefixed with a **+**
            or a **-** to determine the index ordering
        :param background: Allows index creation in the background
        :param drop_dups: Was removed/ignored with MongoDB >2.7.5. The value
            will be removed if PyMongo3+ is used
        """
        if IS_PYMONGO_3 and drop_dups:
            msg = 'drop_dups is deprecated and is removed when using PyMongo 3+.'
            warnings.warn(msg, DeprecationWarning)
        elif not IS_PYMONGO_3:
            kwargs.update({'drop_dups': drop_dups})
        return cls.create_index(key_or_list, background=background, **kwargs)

    @classmethod
    def ensure_indexes(cls):
        """Checks the document meta data and ensures all the indexes exist.

        Global defaults can be set in the meta - see :doc:`guide/defining-documents`

        .. note:: You can disable automatic index creation by setting
                  `auto_create_index` to False in the documents meta data
        """
        background = cls._meta.get('index_background', False)
        drop_dups = cls._meta.get('index_drop_dups', False)
        index_opts = cls._meta.get('index_opts') or {}
        index_cls = cls._meta.get('index_cls', True)
        if IS_PYMONGO_3 and drop_dups:
            msg = 'drop_dups is deprecated and is removed when using PyMongo 3+.'
            warnings.warn(msg, DeprecationWarning)

        collection = cls._get_collection()
        # 746: when connection is via mongos, the read preference is not necessarily an indication that
        # this code runs on a secondary
        if not collection.is_mongos and collection.read_preference > 1:
            return

        # determine if an index which we are creating includes
        # _cls as its first field; if so, we can avoid creating
        # an extra index on _cls, as mongodb will use the existing
        # index to service queries against _cls
        cls_indexed = False

        # Ensure document-defined indexes are created
        if cls._meta['index_specs']:
            index_spec = cls._meta['index_specs']
            for spec in index_spec:
                spec = spec.copy()
                fields = spec.pop('fields')
                cls_indexed = cls_indexed or includes_cls(fields)
                opts = index_opts.copy()
                opts.update(spec)

                # we shouldn't pass 'cls' to the collection.ensureIndex options
                # because of https://jira.mongodb.org/browse/SERVER-769
                if 'cls' in opts:
                    del opts['cls']

                if IS_PYMONGO_3:
                    collection.create_index(fields, background=background, **opts)
                else:
                    collection.ensure_index(fields, background=background,
                                            drop_dups=drop_dups, **opts)

        # If _cls is being used (for polymorphism), it needs an index,
        # only if another index doesn't begin with _cls
        if index_cls and not cls_indexed and cls._meta.get('allow_inheritance'):

            # we shouldn't pass 'cls' to the collection.ensureIndex options
            # because of https://jira.mongodb.org/browse/SERVER-769
            if 'cls' in index_opts:
                del index_opts['cls']

            if IS_PYMONGO_3:
                collection.create_index('_cls', background=background,
                                        **index_opts)
            else:
                collection.ensure_index('_cls', background=background,
                                        **index_opts)

    @classmethod
    def list_indexes(cls):
        """ Lists all of the indexes that should be created for given
        collection. It includes all the indexes from super- and sub-classes.
        """
        if cls._meta.get('abstract'):
            return []

        # get all the base classes, subclasses and siblings
        classes = []

        def get_classes(cls):

            if (cls not in classes and
                    isinstance(cls, TopLevelDocumentMetaclass)):
                classes.append(cls)

            for base_cls in cls.__bases__:
                if (isinstance(base_cls, TopLevelDocumentMetaclass) and
                        base_cls != Document and
                        not base_cls._meta.get('abstract') and
                        base_cls._get_collection().full_name == cls._get_collection().full_name and
                        base_cls not in classes):
                    classes.append(base_cls)
                    get_classes(base_cls)
            for subclass in cls.__subclasses__():
                if (isinstance(base_cls, TopLevelDocumentMetaclass) and
                        subclass._get_collection().full_name == cls._get_collection().full_name and
                        subclass not in classes):
                    classes.append(subclass)
                    get_classes(subclass)

        get_classes(cls)

        # get the indexes spec for all of the gathered classes
        def get_indexes_spec(cls):
            indexes = []

            if cls._meta['index_specs']:
                index_spec = cls._meta['index_specs']
                for spec in index_spec:
                    spec = spec.copy()
                    fields = spec.pop('fields')
                    indexes.append(fields)
            return indexes

        indexes = []
        for klass in classes:
            for index in get_indexes_spec(klass):
                if index not in indexes:
                    indexes.append(index)

        # finish up by appending { '_id': 1 } and { '_cls': 1 }, if needed
        if [(u'_id', 1)] not in indexes:
            indexes.append([(u'_id', 1)])
        if cls._meta.get('index_cls', True) and cls._meta.get('allow_inheritance'):
            indexes.append([(u'_cls', 1)])

        return indexes

    @classmethod
    def compare_indexes(cls):
        """ Compares the indexes defined in MongoEngine with the ones
        existing in the database. Returns any missing/extra indexes.
        """

        required = cls.list_indexes()

        existing = []
        for info in cls._get_collection().index_information().values():
            if '_fts' in info['key'][0]:
                index_type = info['key'][0][1]
                text_index_fields = info.get('weights').keys()
                existing.append(
                    [(key, index_type) for key in text_index_fields])
            else:
                existing.append(info['key'])
        missing = [index for index in required if index not in existing]
        extra = [index for index in existing if index not in required]

        # if { _cls: 1 } is missing, make sure it's *really* necessary
        if [(u'_cls', 1)] in missing:
            cls_obsolete = False
            for index in existing:
                if includes_cls(index) and index not in extra:
                    cls_obsolete = True
                    break
            if cls_obsolete:
                missing.remove([(u'_cls', 1)])

        return {'missing': missing, 'extra': extra}


class DynamicDocument(six.with_metaclass(TopLevelDocumentMetaclass, Document)):
    """A Dynamic Document class allowing flexible, expandable and uncontrolled
    schemas.  As a :class:`~mongoengine.Document` subclass, acts in the same
    way as an ordinary document but has expanded style properties.  Any data
    passed or set against the :class:`~mongoengine.DynamicDocument` that is
    not a field is automatically converted into a
    :class:`~mongoengine.fields.DynamicField` and data can be attributed to that
    field.

    .. note::

        There is one caveat on Dynamic Documents: fields cannot start with `_`
    """

    # The __metaclass__ attribute is removed by 2to3 when running with Python3
    # my_metaclass is defined so that metaclass can be queried in Python 2 & 3
    my_metaclass = TopLevelDocumentMetaclass

    _dynamic = True

    def __delattr__(self, *args, **kwargs):
        """Delete the attribute by setting to None and allowing _delta
        to unset it.
        """
        field_name = args[0]
        if field_name in self._dynamic_fields:
            setattr(self, field_name, None)
            self._dynamic_fields[field_name].null = False
        else:
            super(DynamicDocument, self).__delattr__(*args, **kwargs)


class DynamicEmbeddedDocument(six.with_metaclass(DocumentMetaclass, EmbeddedDocument)):
    """A Dynamic Embedded Document class allowing flexible, expandable and
    uncontrolled schemas. See :class:`~mongoengine.DynamicDocument` for more
    information about dynamic documents.
    """

    # The __metaclass__ attribute is removed by 2to3 when running with Python3
    # my_metaclass is defined so that metaclass can be queried in Python 2 & 3
    my_metaclass = DocumentMetaclass

    _dynamic = True

    def __delattr__(self, *args, **kwargs):
        """Delete the attribute by setting to None and allowing _delta
        to unset it.
        """
        field_name = args[0]
        if field_name in self._fields:
            default = self._fields[field_name].default
            if callable(default):
                default = default()
            setattr(self, field_name, default)
        else:
            setattr(self, field_name, None)


class MapReduceDocument(object):
    """A document returned from a map/reduce query.

    :param collection: An instance of :class:`~pymongo.Collection`
    :param key: Document/result key, often an instance of
                :class:`~bson.objectid.ObjectId`. If supplied as
                an ``ObjectId`` found in the given ``collection``,
                the object can be accessed via the ``object`` property.
    :param value: The result(s) for this key.

    .. versionadded:: 0.3
    """

    def __init__(self, document, collection, key, value):
        self._document = document
        self._collection = collection
        self.key = key
        self.value = value

    @property
    def object(self):
        """Lazy-load the object referenced by ``self.key``. ``self.key``
        should be the ``primary_key``.
        """
        id_field = self._document()._meta['id_field']
        id_field_type = type(id_field)

        if not isinstance(self.key, id_field_type):
            try:
                self.key = id_field_type(self.key)
            except Exception:
                raise Exception('Could not cast key as %s' %
                                id_field_type.__name__)

        if not hasattr(self, '_key_object'):
            self._key_object = self._document.objects.with_id(self.key)
            return self._key_object
        return self._key_object