public function lastTopic($recursive = true)
{
$key = $recursive === true ? 'recursive' : 'current';
if (isset($this->_lastTopic[$key]) === false) {
$this->_lastTopic[$key] = Topic::whereIn('forum_id', $recursive ? $this->allSubforums() : [$this->forum_id])->orderBy('topic_last_post_time', 'desc')->first();
}
return $this->_lastTopic[$key];
}