code: protected function createAggregateQueryWithContentTypeFilter($contentType)
      {
          $qa = $this->createAggregationQuery();
  
          if ($contentType) {
              $qa->match(array('contentType' => $contentType));
          }
  
          return $qa;
      }
docs: @param $contentType
  
  @return \Solution\MongoAggregation\Pipeline\Stage