/**
  * {@inheritdoc}
  */
 protected function normalizeAttributes(AttributeGroupInterface $group)
 {
     $attributes = parent::normalizeAttributes($group);
     return implode(',', $attributes);
 }
 /**
  * {@inheritdoc}
  */
 public function normalize($object, $format = null, array $context = [])
 {
     $data = parent::normalize($object, $format, $context);
     $data['attributes'] = implode(',', $data['attributes']);
     return $data;
 }