code: def dereference(self, session, ref, allow_none=False):
          
          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)
docs: Dereference an ObjectID to this field's underlying type