'A value to alter sorting of entities in output_entities entity_generator.
Will only be called if calculate_sort_key_from_entity is true.
Args:
entity: A datastore.Entity.
Returns:
A value to store in the intermediate sqlite table. The table will later
be sorted by this value then by the datastore key, so the sort_key need
not be unique.'

def sort_key_from_entity(self, entity):

    return ''
