code: function findValue(payload, instanceKey) {
      for (let key in payload) {
          if (key !== instanceKey) {
              return payload[key];
          }
      }
  
      // if we don't have a value, throw an error because the payload is invalid.
      /* istanbul ignore next */
      throw new Error('Failed to mutate instance, no value found in payload.', payload);
  }
docs: helper function to find the payload value