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

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

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

        :rtype: :class:`.entity_pb2.Entity`
        :returns: The newly created entity protobuf that will be
                  updated and sent with a commit.
        """
        new_mutation = _datastore_pb2.Mutation()
        self._mutations.append(new_mutation)
        return new_mutation.insert