public function get_links()
 {
     if (is_null($this->links)) {
         $this->links = LinkRepository::instance()->find_by_category($this);
     }
     return $this->links;
 }
 public function find_link_by_url($url)
 {
     $c_id = $this->c_id;
     $session_id = $this->session_id;
     $repo = LinkRepository::instance();
     $link = $repo->find_one_by_course_and_url($c_id, $session_id, $url);
     return $link;
 }