code: public function onKernelController(FilterControllerEvent $event)
      {
          if (!is_array($controller = $event->getController())) {
              return;
          }
  
          $request = $event->getRequest();
  
          $method = new \ReflectionMethod($controller[0], $controller[1]);
  
          if ($this->isHydraOperation($method)) {
              $request->attributes->set('__hydra_serialize', true);
  
              // TODO Add support for serialization groups and other serialization formats
          }
      }
docs: Marks request that whose controller return value should be serialized
  by the Hydra serializer
  
  @param FilterControllerEvent $event A FilterControllerEvent instance