public function clear_assessments() {
        global $DB;

        $submissions = $this->get_submissions();
        if (empty($submissions)) {
            // no money, no love
            return;
        }
        $submissions = array_keys($submissions);
        list($sql, $params) = $DB->get_in_or_equal($submissions, SQL_PARAMS_NAMED);
        $sql = "submissionid $sql";
        $DB->set_field_select('workshop_assessments', 'grade', null, $sql, $params);
        $DB->set_field_select('workshop_assessments', 'gradinggrade', null, $sql, $params);
    }