'Return the ids (as a tuple) for the values that the field ``name`` has on the book
identified by ``book_id``. If there are no values, or no such book, or
no such field, an empty tuple is returned.'

@read_api
def field_ids_for(self, name, book_id):

    try:
        return self.fields[name].ids_for_book(book_id)
    except (KeyError, IndexError):
        return ()
