protected function get_grading_userid_list($cached = false, $useridlistid = '') {
        if ($cached) {
            if (empty($useridlistid)) {
                $useridlistid = $this->get_useridlist_key_id();
            }
            $useridlistkey = $this->get_useridlist_key($useridlistid);
            if (empty($SESSION->mod_assign_useridlist[$useridlistkey])) {
                $SESSION->mod_assign_useridlist[$useridlistkey] = $this->get_grading_userid_list(false, '');
            }
            return $SESSION->mod_assign_useridlist[$useridlistkey];
        }
        $filter = get_user_preferences('assign_filter', '');
        $table = new assign_grading_table($this, 0, $filter, 0, false);

        $useridlist = $table->get_column_data('userid');

        return $useridlist;
    }