code: public function reverseTransform($id)
      {
          if (!$id) {
              return null;
          }
          
          if ($id instanceof CustomFieldsGroup) {
              throw new TransformationFailedException(sprintf(
                    'The transformation failed: the expected argument on '
                    . 'reverseTransform is an object of type int,'
                    . 'Chill\CustomFieldsBundle\Entity\CustomFieldsGroup, '
                    . 'given', gettype($id)));
          }
  
          $customFieldsGroup = $this->om
              ->getRepository('ChillCustomFieldsBundle:customFieldsGroup')->find($id)
          ;
  
          if (null === $customFieldsGroup) {
              throw new TransformationFailedException(sprintf(
                  'Le group avec le numéro "%s" ne peut pas être trouvé!',
                  $id
              ));
          }
  
          return $customFieldsGroup;
      }
docs: Transforms a string (id) to an object (CustomFieldsGroup).
  
  @param  string $id
  @return CustomFieldsGroup|null
  @throws TransformationFailedException if object (report) is not found.