/**
  * {@inheritdoc}
  */
 public function loadMultiple(array $ids = NULL)
 {
     $entities = parent::loadMultiple($ids);
     $uuid_key = $this->entityType->getKey('uuid');
     array_walk($entities, function (ContentEntityInterface $rdf_entity) use($uuid_key) {
         // The ID of 'rdf_entity' is universally unique because it's a URI. As
         // the backend schema has no UUID, ID is reused as UUID.
         $rdf_entity->set($uuid_key, $rdf_entity->id());
     });
     return $entities;
 }