private static function assign_user_mappings() {
        return new external_single_structure(
            array (
                'assignmentid'    => new external_value(PARAM_INT, 'assignment id'),
                'mappings'   => new external_multiple_structure(new external_single_structure(
                        array(
                            'id'     => new external_value(PARAM_INT, 'user mapping id'),
                            'userid' => new external_value(PARAM_INT, 'student id')
                        )
                    )
                )
            )
        );
    }