'buyItem': async (req, res, next) => {
  const itemId = req.params.itemId;
  const userId = req.headers['current-user-id'];

  const orderRecord = await Order.find(itemId, userId)//do mongodb search
  if (orderRecord && orderRecord.status === 'paid') {//second request  detect no orderRecord exists beacase first request haven't create order yet
    return res.status(400).json({
      'code': 101,
      'message': 'you have already bought it',
      'result': {}
    })
  }
  // fisrt request goes here
  const orderNew = await Order.create(itemId, userId)//do mongodb create
  await User.deduct(userId, item.price);

}