code: public function getTopicById($id)
      {
          $select = $this->getSelect()->where(array(
              'id' => $id
          ));
          $ret = $this->select($select)->current();
          /**
           *
           * @return \Zd2Forum\Options\ModuleOptions
           */
          $options = $this->getServiceLocator()->get('Zf2Forum\ModuleOptions');
          $funcName = "get" . $options->getUserColumn();
          $user = $this->getServiceLocator()
              ->get("Zf2Forum_user_mapper")
              ->findById($ret->getUserId());
          $ret->user = $user->$funcName();
          return $ret;
      }
docs: getTopicById
  
  @param int $int
  @return TopicInterface