Express.js, Mongoose: Populate returns null - node.js

Question

I'm learning express.js and mongoDB and want to create a functionality, where user should be able add products.
My code contains 2 models, User and Product. User has a reference to Product. In User query when i try to populate Product', null is returned.
I'm testing it with postman, and the result is null in cart.
I am a beginner and don't understand how to solve the issue.
user schema
import mongoose, { Document } from 'mongoose'
import { ProductDocument } from './Products'
export type UserDocument = Document & {
firstName: string;
lastName: string;
password: string;
email: string;
isAdmin: boolean;
cart: ProductDocument[];
resetLink: string;
}
const userSchema = new mongoose.Schema({
firstName: {
type: String,
required: true,
max: 64,
},
lastName: {
type: String,
required: true,
max: 64,
},
password: {
type: String,
required: true,
min: 6,
},
email: {
type: String,
required: true,
lowercase: true,
trim: true,
},
isAdmin: {
type: Boolean,
default: false,
},
cart: [
{
type: mongoose.Schema.Types.ObjectId,
ref: 'Products',
},
],
resetLink: {
data: String,
default: '',
},
})
export default mongoose.model<UserDocument>('Users', userSchema)
product schema
import mongoose, { Document } from 'mongoose'
export type ProductDocument = Document & {
name: string;
description: string;
categories: string[];
variants: string[];
sizes: number[];
}
const productSchema = new mongoose.Schema({
name: {
type: String,
required: true,
index: true,
},
description: {
type: String,
},
categories: [String],
variants: [String],
sizes: [Number],
})
export default mongoose.model<ProductDocument>('Products', productSchema)
and the controller for responsible for populating
export const getProduct = async (req: Request, res: Response) => {
try {
const { productId } = await req.body
const productBought = await Products.findOne({ _id: productId }).then(
(product) => {
return product
}
)
console.log(productBought)
const updated = await Users.findOneAndUpdate(
{ _id: req.params.userId },
{ $push: { cart: productBought } },
{ new: true }
)
.populate('cart')
.exec()
return res.json(updated)
} catch (error) {
return res.status(404).json({ message: 'does not work' })
}
}
output from postman
{
"isAdmin": false,
"cart": [
null,
null
],
"_id": "5f894b3c06b4a108f8d9a7ab",
"firstName": "John",
"lastName": "Doe",
"password": "$2b$08$rA0/r8iVBWeSyyerPDpPWO.ztgouQoseX0QFBZ.mlPgb6tELlrhpy",
"email": "john#gmail.com",
"__v": 0
}

Answer 1

your "cart" like[ObjectId,ObjectId], but your "productBought" like[{},{}].
try let cart = [];productBought.forEach((v,k) => {cart[k] = v._id});
and use { $push: { cart: { $each: cart } },