public function get_course_ids($categories) {
        $courseids = array();
        foreach ($categories as $key=>$cat) {
            $courseids[$key] = $cat->course;
            if (!empty($cat->children)) {
                $courseids = array_merge($courseids, $this->get_course_ids($cat->children));
            }
        }
        return $courseids;
    }