public function add()
 {
     if (!Input::has('comment')) {
         return Response::json(array('statusCode' => 202, 'error' => 'invalid param(s).'));
     }
     $comment = new Comment();
     $comment->text = Input::get('comment');
     $comment->created_at = (new DateTime())->format('Y-m-d H:i:s');
     $service = new CommentService($comment);
     if ($service->save()) {
         return Response::json(array('statusCode' => 200, 'message' => 'success'));
     }
     return Response::json(array('statusCode' => 202, 'error' => 'database error occur.'));
 }