Adds a new mutation for an entity with a completed key.

        :rtype: :class:`.entity_pb2.Entity`
        :returns: The newly created entity protobuf that will be
                  updated and sent with a commit.

def _add_complete_key_entity_pb(self):
        """Adds a new mutation for an entity with a completed key.

        :rtype: :class:`.entity_pb2.Entity`
        :returns: The newly created entity protobuf that will be
                  updated and sent with a commit.
        """
        # We use ``upsert`` for entities with completed keys, rather than
        # ``insert`` or ``update``, in order not to create race conditions
        # based on prior existence / removal of the entity.
        new_mutation = _datastore_pb2.Mutation()
        self._mutations.append(new_mutation)
        return new_mutation.upsert