Populating the cart doesnt work (mongoose) - node.js

Question

I have been trying to populate cart products in user's cart. Below is my code.
const express = require('express')
const mongoose = require('mongoose')
const bodyParser = require('body-parser')
const app = express()
app.use(bodyParser.urlencoded({extended:false}))
const Schema = mongoose.Schema;
var userSchema = new Schema({
userName : {
type : String,
default : 'UserName'
},
cart : [ { type : mongoose.Schema.Types.ObjectId , ref : 'Cart' } ]
})
var cartSchema = new Schema({
productNameofUser : {
type : String,
default : 'Product'
}
})
var UserModel = mongoose.model('User' , userSchema)
var CartModel = mongoose.model('Cart' , cartSchema)
app.get('/users',(req,res)=>{
UserModel.find().populate('cart').exec((err,result)=>{
res.send(result)
})
})
app.post('/createUser',(req,res)=>{
let newUser = new UserModel()
newUser.save((err,result)=>{
console.log(result)
res.send(result)
})
})
app.get('/products',(req,res)=>{
CartModel.find((err,result)=>{
res.send(result)
})
})
app.post('/createProduct',(req,res)=>{
let newProduct = new CartModel()
newProduct.save((err,result)=>{
res.send(result)
})
})
app.listen(3000,()=>{
console.log('running')
mongoose.connect('mongodb://127.0.0.1:27017/populate' , { useNewUrlParser : true })
})
what I am trying to do is, There is a carts collection and there is another collection of users. Every user has a cart which i am trying to populate from the cart collections.
If you want to test the code, first create the user using the post link 'http://localhost:3000/createUser/' via postman and the create cart Products using 'http://localhost:3000/createProduct/' via postman. Then whenever i tried to fetch the users 'http://localhost:3000/users/' , the cart array stays empty no matter what i try.
Please figure where i am going wrong

Answer 1

Update your Schema defination as:
const Schema = mongoose.Schema;
var userSchema = new Schema({
userName : {
type : String,
default : 'UserName'
},
cart : [ { type : Schema .ObjectId , ref : 'Cart' } ]
})
And try this again:
app.get('/users',(req,res)=>{
UserModel.find().populate('cart').exec((err,result)=>{
res.send(result)
})
})
Let me know, if this works now.