code: def _check_database_id(database_id):
      
      if database_id != u"":
          msg = _DATABASE_ID_TEMPLATE.format(database_id)
          raise ValueError(msg)
docs: Make sure a "Reference" database ID is empty.
  
      :type database_id: unicode
      :param database_id: The ``database_id`` field from a "Reference" protobuf.
  
      :raises: :exc:`ValueError` if the ``database_id`` is not empty.