public function unenrol_user(stdClass $instance, $userid) {
        global $DB;

        // Get the tool associated with this instance. Note - it may not exist if we have deleted
        // the tool. This is fine because we have already cleaned the 'enrol_lti_users' table.
        if ($tool = $DB->get_record('enrol_lti_tools', array('enrolid' => $instance->id), 'id')) {
            // Need to remove the user from the users table.
            $DB->delete_records('enrol_lti_users', array('userid' => $userid, 'toolid' => $tool->id));
        }

        parent::unenrol_user($instance, $userid);
    }