update products with the same id - node.js

Question

I got products with the option to choose size,
So If I add 'X' product to the cart with sizes 4 and 5 it will add 2 items to the cart like this :
My goal is when you succeed buying that items from your cart the sizes that you just bought will be removed from main product page.
It works good only if I trying to buy 2 different items.
If I will try to buy 2 same items with different size only the first size will be filtered and I will get this error :
(node:21336) UnhandledPromiseRejectionWarning: VersionError: No matching document found for id "62bee4ce92e7c57a195686ae" version 0 modifiedPaths "size"
this is the success buy code :
createOrder: async (_, {}, context) => {
const userAuth = await auth(context);
const cart = await Cart.findOne({ userId: userAuth._id });
cart.cartProducts.forEach(async (p) => { // Here's the filtering product functionallity
const products = await Product.findById(p.productId);
if (products) {
products.size = products.size.filter((s) => s !== +p.size);
}
await products.save();
});
if (!cart) {
throw new UserInputError('No available order!');
}
const newOrder = new Order({
orderProducts: cart.cartProducts,
purchasedBy: userAuth._id,
datePurchased: new Date().toISOString(),
});
await newOrder.save();
return newOrder;
},
as I said it works only if you add 2 different items to your cart.
edited:
Now I am getting product.save is not a function
const userAuth = await auth(context);
const cart = await Cart.findOne({ userId: userAuth._id });
const products = await Product.find({
_id: cart.cartProducts.map((c) => c.productId),
});
cart.cartProducts.forEach(async (p) => {
if (products) {
products.map((product) => {
return (product.size = product.size.filter(
(size) => size !== +p.size
));
});
}
});
await products.save();

Answer 1

The issue is that you are using findById which will return the first matching document. So that is why when you add same product with different size in cart. It will always pick the first matching id of the product. You can try using find operator as it returns the list matching the condition.
Product.find({_id:p.productId});
It will return all of the products instead of matching only the first one.