public function delete_instance() {
        global $DB;
        $grades = $DB->get_records('assign_grades', array('assignment'=>$this->assignment->get_instance()->id), '', 'id');
        if ($grades) {
            list($gradeids, $params) = $DB->get_in_or_equal(array_keys($grades), SQL_PARAMS_NAMED);
            $DB->delete_records_select('assignfeedback_editpdf_annot', 'gradeid ' . $gradeids, $params);
            $DB->delete_records_select('assignfeedback_editpdf_cmnt', 'gradeid ' . $gradeids, $params);
        }
        return true;
    }