/**
  * Begin a transaction and return it's reference id
  *
  * @param bool $bol_cross_group
  * @return string
  * @throws \Exception
  */
 public function beginTransaction($bol_cross_group = FALSE)
 {
     if ($bol_cross_group) {
         throw new \Exception("Cross group transactions not supported over JSON API");
     }
     $obj_request = new \Google_Service_Datastore_BeginTransactionRequest();
     /** @var \Google_Service_Datastore_BeginTransactionResponse $obj_response */
     $obj_response = $this->obj_datasets->beginTransaction($this->str_dataset_id, $obj_request);
     return $obj_response->getTransaction();
 }