错误：订单验证失败：productId：必填路径`productId`

Question

我正在尝试使用带有productId的nodejs和mongodb保存订单，但是在这里我收到一条错误消息订单验证失败：productId：需要路径productId。” 我在Postman中执行POST方法，请帮助我解决此问题

路线order.js代码

const express = require('express');
const router = express.Router();
const mongoose = require('mongoose');

const Order = require('../models/order');


router.post('/', (req, res, next) =>{
const order = new Order({
        _id: new mongoose.Types.ObjectId(),
        quantity: req.body.quantity,
        productId: req.body.productId
});
order.save().then(result => {
    console.log(result);
    res.status(201).json({
        message: 'Created order successfully',
        createdProduct: {
            quantity: result.quantity,
            productId: result.productId,
            _id: result._id,
            request:{
                type: 'GET',
                url:  'http://localhost:3000/orders/' + result._id
            }
        }
    });
})
.catch(err => {
    console.log(err);
    res.status(500).json({
        error: err
    });
});
});
module.exports = router;

建模order.js

const mongoose = require('mongoose');
const orderSchema = mongoose.Schema({
_id: mongoose.Schema.Types.ObjectId,
productId: { type: mongoose.Schema.Types.ObjectId, ref: 'Product', required: true},
quantity: { type: Number, default: 1}
});
module.exports = mongoose.model('Order', orderSchema);

请先让我知道我如何克服这个问题