Dereference an ObjectID to this field's underlying type

def dereference(self, session, ref, allow_none=False):
        """ Dereference an ObjectID to this field's underlying type """
        ref = DBRef(id=ref, collection=self.type.type.get_collection_name(),
                    database=self.db)
        ref.type = self.type.type
        return session.dereference(ref, allow_none=allow_none)