code: function _result(err, dbName, docId, message) {
      var result = {
        id: util.format('%s%s%s', dbName, (docId) ? '/' : '', docId || ''),
        message: message || util.format('%s (%s)', err.error, err.status_code)
      };
      if (err) {
        result.error = err;
      }
      return result;
    }
docs: construct result object, the existence of error field indicates whether the result is a success or not