/**
  * @param  string         $subscriber_id
  * @return DbMessageQuery $this
  */
 public function withSubscriber($subscriber_id = null)
 {
     if ($subscriber_id === null) {
         $modelClass = $this->modelClass;
         $t = $modelClass::tableName();
         $this->andWhere("{$t}.subscription_id IS NULL");
     } else {
         $this->innerJoinWith('subscription');
         $this->andWhere(DbSubscription::tableName() . '.subscriber_id=:subscriber_id', [':subscriber_id' => $subscriber_id]);
     }
     return $this;
 }