Business logic works but Mongoose save doesn't - node.js

Question

Hi I'm trying to save data from my shopping cart app to MongoDB database. In my cart controller folder, there are getAllProducts, addOneProduct, removeOneProduct and updateOneProducts methods. All of them run perfectly fine except for updateOneProducts.
The console.log shows:
GET all product { id: [1,2,4], qty: [1,1,1] }
UPDATE one product { id: [1,2,4], qty: [1,1,2] }
which indicate that the business logic works. However, the document is not updated on mLab.
User model
const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const userSchema = new Schema({
username: String,
userID: String,
picture: String,
products: {
id: [Number],
qty: [Number],
}
});
const Users = mongoose.model('User', userSchema);
module.exports = Users;
updateOneProduct
const extractProduct = (user) => {
const product = {};
product.id = user.products.id;
product.qty = user.products.qty;
return product;
}
// if user is logged in n changes the qty of a product, update the qty in database
const updateOneProduct = (req, res) => {
// if (!req.user) return res.status(401).json({ error: 'Unauthorized' });
const userId = req.user._id;
const event = req.body.event;
const productId = req.body.id;
return Users.findById(userId)
.then(user => {
const qty = user.products.qty;
const index = user.products.id.indexOf(productId);
if (index === -1) {
throw new Error(`Product doesn't exist!`);
}
const regex = /^[0-9\b]+$/;
if (event === 'plus') {
qty[index] += 1;
} else if (event === 'minus') {
qty[index] -= 1;
} else if (regex.test(event) && event !== '0') {
qty[index] = Number(event);
}
if (qty[index] === 0) {
user.products.id.splice(index, 1);
qty.splice(index, 1);
}
return user.save();
})
.then(user => {
const product = extractProduct(user);
console.log('UPDATE one product', product);
return res.status(200).json(product);
})
.catch(error => res.status(400).json({ error }));
}

Answer 1

So I basically cloned user.products.qty array with const qty = [...user.products.qty], do operations on it and replace user.products.qty with it using user.products.qty = qty. Lastly, I save it with return user.save().
For some reason, I can't mutate subdocument project and save it. I have to replace the whole subdocument user.products then save for it to work. I think it might also have something to do with this being a PUT request.
// if user is logged in n changes the qty of a product, update the qty in database
const updateOneProduct = (req, res) => {
// if (!req.user) return res.status(401).json({ error: 'Unauthorized' });
const userId = req.user._id;
const event = req.body.event;
const productId = req.body.id;
return Users.findById(userId)
.then(user => {
const qty = [...user.products.qty];
const index = user.products.id.indexOf(productId);
if (index === -1) {
throw new Error(`Product doesn't exist!`);
}
const regex = /^[0-9\b]+$/;
if (event === 'plus') {
qty[index] += 1;
} else if (event === 'minus') {
qty[index] -= 1;
} else if (regex.test(event) && event !== '0') {
qty[index] = Number(event);
}
if (qty[index] === 0) {
user.products.id.splice(index, 1);
qty.splice(index, 1);
}
user.products.qty = qty;
return user.save();
})
.then(user => {
console.log('UPDATE - After save promise', user.products);
return res.status(200).json(user.products);
})
.catch(error => res.status(400).json({ error }));
}