public function getNewMessagesForConversation($conversationId, $lastMessageTimestamp = null)
 {
     if (($conversation = $this->getConversation($conversationId)) === null) {
         return array();
     }
     if (empty($lastMessageTimestamp)) {
         $lastMessageTimestamp = time();
     }
     $result = array();
     $messageList = $this->messageDao->findConversationMessagesByLastMessageTimestamp($conversation->id, $lastMessageTimestamp);
     foreach ($messageList as $message) {
         $result[] = $this->getMessageDataForApi($message);
     }
     return $result;
 }