public function addComment($comment)
    {
        if(Yii::$app->params['commentNeedApproval'])
            $comment->status = Comment::STATUS_PENDING;
        else
            $comment->status = Comment::STATUS_APPROVED;
        // set model and parent_id (parent model ID)
        $comment->model = self::COMMENT_FOR;
        $comment->parent_id = $this->id;
        $comment->user_id = \Yii::$app->user->id;
        if($comment->thread == '-')
            // set new thread
            $comment->thread = time() . uniqid();
        else
            // unmark previous comment
            Yii::$app->db->createCommand("UPDATE {{%comment}} SET last=0 WHERE thread='{$comment->thread}' AND last=1")->execute();
        // mark only last comment in a thread
        $comment->last = 1;
        if($comment->save()) {
            if(Yii::$app->params['notifyResponsibleAboutComment'])
                $comment->trigger(Comment::EVENT_JUST_ADDED);
            $comment->content = '';
            return true;
        } else
            return false;
    }