public function link_url($field, $id = null)
    {
        if ($qpos = strpos($this->url, '?')) {
            $url = substr($this->url, 0, $qpos);
        } else {
            $url = $this->url;
        }

        if ($id) {
            $url = preg_replace('/[^\/]+$/', $id, rtrim($url, "/"));
        }

        return $url."/".$field;
    }