[node.js]Angular MongoDB 在为用户添加新购物车时出现问题

 This is what I get:
[
  {
    _id: ObjectId("658bfdd2e06b20c39f8816f8"),
    userId: ObjectId("6588baa65a346036b4479398"),
    product: [ { _id: ObjectId("658bfdd2e06b20c39f8816f9") } ],
    active: true,
    modifiedOn: ISODate("2023-12-27T10:34:58.900Z"),
    createdAt: ISODate("2023-12-27T10:34:58.932Z"),
    updatedAt: ISODate("2023-12-27T10:34:58.932Z"),
   __v: 0
   }
 ]
  
  This is my request
   product: {_id: "651352899db20fb85059c210", name: "Pink To Slick",
   size: "S",…}
   description: "Lorem Ipsum is simply dummy text of the printing and 
   typesetting industry. Lorem Ipsum has been the industry's standard dummy 
  text ever since the 1500s."
   imageUrl: "http://localhost:4200/assets/PinkToSlick.png"
   name: "Pink To Slick"
   price: 299
   size: "S"
   _id: "651352899db20fb85059c210"

 

 var express = require('express');
 const mongoose = require("mongoose");
 const Products      = require('../backend-search/Products.cjs');
 const User = require('../../app/backend/users/user.model')
 const productSchema = new mongoose.Schema({

 name:{
   type: String,
   },
   size:{type: String}, 

   imageUrl:{ type:String},

   price:{type:Number},

  description:{
   type:String
  }

 });

 const cartitemSchema = new mongoose.Schema({
    userId: {
    type: mongoose.Schema.Types.ObjectId,
    ref: "User"
  },

     product: [ productSchema],

  
  


   active: {
   type: Boolean,
   default: true
 },
   modifiedOn: {
   type: Date,
   default: Date.now
 }
},
{ timestamps: true },



 );

      module.exports =
      mongoose.model('Cartitem', cartitemSchema, 'cartitems');


      
//Here is my code

     router.post('/getProducts/addtocart',   async(req, res)=>{


     const { name, description,size, imageUrl, price} = req.body

    _id = req.params._id;

     //hard coded for now
    userId =({"_id" : ObjectId("6588baa65a346036b4479398")});


   try{
          // -------Get users Cart ------
        let cart = await Cartitem.findOne({userId:userId});



   if(cart){      //means cart is not null

  //-----If cart exist for user-------
   let indexFound = cart.product.findIndex(product => {
      product._id == _id
     }


  //----------check if product exist
   if (indexFound > -1) {
 /*product exists in the cart, update the quantity *** Might have to add 
 this*/

 let productItem = cart.product[indexFound];
  console.log("Product Item" + "" + productItem);
  cart.product[indexFound] = productItem;

 }

mongoose.connect(mongoURI, async function(err, db){
  assert.ok("1" == 1 );  

  db.collection('cartitems').updateOne({_id:_id},{


  $push:{
    product:{
      _id:_id,
      name:name, size:size, imageUrl:imageUrl, price:price
    }
  }

})


   
   cart = await cart.save(); 
   console.log("What is cart" + "" + "" + cart );
   return res.status(201).send(cart);

  });
     
  } 
  //cart stops here

  else{   

 //no cart for user, create new cart
   const newCartitem  =  new Cartitem({
    userId,
    product: [{
      name, 
      size,
      imageUrl,
      price,
      description
    }]   


    
  });

   const purchase =  newCartitem.save()
 

  
  
   return res.status(201).send({purchase});

  }

  } catch (err) {
    console.log(err);
    res.status(500).send("Something went wrong");
  }


});