code: def dereference(self, dbref, session=None, **kwargs):
          
          if not isinstance(dbref, DBRef):
              raise TypeError("cannot dereference a %s" % type(dbref))
          if dbref.database is not None and dbref.database != self.__name:
              raise ValueError("trying to dereference a DBRef that points to "
                               "another database (%r not %r)" % (dbref.database,
                                                                 self.__name))
          return self[dbref.collection].find_one(
              {"_id": dbref.id}, session=session, **kwargs)
docs: Dereference a :class:`~bson.dbref.DBRef`, getting the
          document it points to.
  
          Raises :class:`TypeError` if `dbref` is not an instance of
          :class:`~bson.dbref.DBRef`. Returns a document, or ``None`` if
          the reference does not point to a valid document.  Raises
          :class:`ValueError` if `dbref` has a database specified that
          is different from the current database.
  
          :Parameters:
            - `dbref`: the reference
            - `session` (optional): a
              :class:`~pymongo.client_session.ClientSession`.
            - `**kwargs` (optional): any additional keyword arguments
              are the same as the arguments to
              :meth:`~pymongo.collection.Collection.find`.
  
          .. versionchanged:: 3.6
             Added ``session`` parameter.