code: public function makeDefaultAction(Request $request)
      {
          
          $form = $this->createMakeDefaultForm(null);
          $form->handleRequest($request);
  
          $cFGroupId = $form->get('id')->getData();
  
          $em = $this->getDoctrine()->getManager();
  
          $cFGroup = $em->getRepository('ChillCustomFieldsBundle:CustomFieldsGroup')->findOneById($cFGroupId);
  
          if(!$cFGroup) {
              throw $this
                    ->createNotFoundException("customFieldsGroup not found with "
                          . "id $cFGroupId");
          }
  
          $cFDefaultGroup = $em->getRepository('ChillCustomFieldsBundle:CustomFieldsDefaultGroup')
              ->findOneByEntity($cFGroup->getEntity());
  
          if($cFDefaultGroup) {
              $em->remove($cFDefaultGroup);
              $em->flush(); /*this is necessary, if not doctrine
               * will not remove old entity before adding a new one, 
               * and this leads to violation constraint of unique entity
               * in postgresql
               */
          } 
          
          $newCFDefaultGroup = new CustomFieldsDefaultGroup();
          $newCFDefaultGroup->setCustomFieldsGroup($cFGroup);
          $newCFDefaultGroup->setEntity($cFGroup->getEntity());
  
          $em->persist($newCFDefaultGroup);
          $em->flush();
          
          $this->addFlash('success', $this->get('translator')
                    ->trans("The default custom fields group has been changed"));
  
          return $this->redirect($this->generateUrl('customfieldsgroup'));
      }
docs: Set the CustomField Group with id $cFGroupId as default