docs: def create_db(self):
          """
          "   Create a db file for model if there is no db.
          "   User need to prepare thier own xxx.json.factory.
          """
          if self.db_type != "json":
              raise RuntimeError("db_type only supports json now")
  
          if os.path.exists(self.json_db_path):
              return False
  
          if os.path.exists(self.factory_json_db_path):
              with self.db_mutex:
                  shutil.copy2(
                      self.factory_json_db_path, self.json_db_path)
              return True
  
          _logger.debug(
              "*** NO such file: %s" % self.factory_json_db_path)
          raise RuntimeError("No *.json.factory file")