code: def reference(self):
      
      if self.__reference is None:
        self.__reference = _ConstructReference(self.__class__,
                                               pairs=self.__pairs,
                                               app=self.__app,
                                               namespace=self.__namespace)
      return self.__reference
docs: Return the Reference object for this Key.
  
      This is a entity_pb.Reference instance -- a protocol buffer class
      used by the lower-level API to the datastore.
  
      NOTE: The caller should not mutate the return value.