/**
  * Generate an import key that can be used in background imports.
  *
  * @param Batch $batch
  *
  * @return string
  *
  * @throws Exception
  */
 public function generate_import_key(Batch $batch)
 {
     if (!$batch->get_id() || !$batch->get_modified_gmt()) {
         throw new Exception('Failed generating batch import key.');
     }
     $key = md5($batch->get_id() . '-' . $batch->get_modified_gmt() . '-' . rand(0, 100000));
     update_post_meta($batch->get_id(), '_sme_import_key', $key);
     return $key;
 }