code: def _write_version(self, data, model):
          
          vdata = {'data': data,
                   'key': model.key,
                   'model': model.Meta.bucket_name,
                   'timestamp': time.time()}
          obj = version_bucket.new(data=vdata)
          obj.add_index('key_bin', model.key)
          obj.add_index('model_bin', vdata['model'])
          obj.add_index('timestamp_int', int(vdata['timestamp']))
          obj.store()
          return obj.key
docs: Writes a copy of the objects current state to write-once mirror bucket.
  
          Args:
              data (dict): Model instance's all data for versioning.
              model (instance): Model instance.
  
          Returns:
              Key of version record.
              key (str): Version_bucket key.