'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.'

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
