Field id expected a number but got <Cart: John's cart>

Question

I have this view I would like to create a cart or update the entries if the cart is already created and if a product is already in the cart then just update its quantity. I am getting an error saying Field id expected a number but got <Cart: John's cart>. Why is the id field even mentioned when I specifically put cart to filter, because I do not have the id of the Entry

class CreateCart(generics.CreateAPIView):
    permission_classes = [permissions.IsAuthenticated]
    serializer_class = CartSerializer   
    def post(self, request):
        user = User.objects.get(id=request.data.get("user"))
        total = request.data.get("carttotal")
        usercart = Cart.objects.update_or_create(user=user, cart_total=total)
        products = request.data.getlist("products")
        for product in products:
            obj = json.loads(product)
            id = obj.get("id")
            qty = obj.get("qty")
            product = Product.objects.get(id=id)
            exists = Entry.objects.all().filter(cart=usercart, product=product)
            if exists:
                exists.quantity = qty
            else:
                Entry.objects.create(product=product, cart=usercart, quantity=qty)
        return Response("Items added or updated", status=HTTP_201_CREATED)

This line Django is complaining about:

exists = Entry.objects.all().filter(cart=usercart, product=product)

Answer 1

At first, the update_or_create() method returns a tuple containing the object and a boolean value indicating whether it was created or updated. So, you should unpack the tuple to get the actual Cart instance as:

usercart, created = Cart.objects.update_or_create(user=user, cart_total=total)