我尝试将产品添加到存储在mongodb中的购物车，但出现“TypeError：无法读取未定义的属性（阅读“查找”）”将发生，导致它不添加任何产品到购物车。我已经花了一整天试图让这个工作没有运气，任何帮助将不胜感激。

const express = require('express');
const Carts = require('../repo/carts');
// const cartShowTemplate = require('../views/carts/show');
const router = express.Router();
let cart;
router.post('/cart/products', async (req, res) => {
  if (!req.session.cartId) {
    cart = await Carts.create({ items: [] });
    req.session.cartId = cart._id;
  } else {
    cart = await Carts.find(req.session.cartId);
  }
  console.log(cart);
    // look thru cart and find item with id property equal to req.body.productId 
      property
    //if item exists increment by 1 and save cart
    //add new product to items array
  const existingItem = cart.items.find(item => item._id  === req.body.productId)
  if (existingItem) {
    existingItem++;
  } else {
    cart.items.push({ id: req.body.productId, quantity: 1 });
  } 
  await Carts.updateOne(cart._id, {
    items: cart.items
  });
  res.send('product added to cart!!');
});
module.exports = router;

carts mongodb模式

const mongoose = require('mongoose');
const cartSchema = new mongoose.Schema({
  _id: String,
  items: [
    { quantity: Number, _id: String },
  ] 
});
const Carts = new mongoose.model('Carts', cartSchema);
module.exports = Carts;