code: protected function findLastVersionInLanguage(Stage $qa)
      {
          $elementName = 'node';
          $qa->sort(array('createdAt' => 1));
          $qa->group(array(
              '_id' => array('nodeId' => '$nodeId', 'language' => '$language'),
              $elementName => array('$last' => '$$ROOT')
          ));
  
          return $this->hydrateAggregateQuery($qa, $elementName);
      }
docs: @param Stage $qa
  
  @return array