/**
  * Unsubscribe user
  *
  * @param User $user
  * @return boolean
  */
 function unsubscribeUser(User $user)
 {
     $subscription = MessageSubscriptions::findById(array('message_id' => $this->getId(), 'user_id' => $user->getId()));
     // findById
     if ($subscription instanceof MessageSubscription) {
         return $subscription->delete();
     } else {
         return true;
     }
     // if
 }