code: public function reverseTransform($value)
      {
          $sites = array();
  
          foreach ($value as $site) {
              $sites[] = array('siteId' => $site->getSiteId());
          }
  
          return $sites;
      }
docs: @param SiteInterface $value
  
  @return string
  
  @throws TransformationFailedException When the transformation fails.