我正在尝试做一个购物车系统，用户可以将产品添加到购物车中，如果它已经在那里，我会添加其中一个，但我一直得到错误“'tuple' object has no attribute 'quantity' django”。

class OrderItem(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    item = models.ForeignKey(Product, on_delete=models.CASCADE)
    quantity = models.IntegerField(default=1)
    cart = models.BooleanField(default=True)
    order = models.BooleanField(default=False)

class Orders(models.Model) :
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    items = models.ManyToManyField(OrderItem)

def add_to_cart(request, pk) :
    item = get_object_or_404(Product, pk=pk)
    order_item = OrderItem.objects.get_or_create(
        item = item,
        user = request.user,
        order = False
    )
    order_qs = Orders.objects.filter(user=request.user)

    if order_qs.exists():
        order = order_qs[0]

        if order.items.filter(item__pk=item.pk).exists() :
            order_item.quantity += 1
            order_item.save()
            return redirect("product-detail", pk = pk)
        else:
            order.items.add(order_item)
            return redirect("product-detail", pk = pk)
    else:
        order = Orders.objects.create(user=request.user)
        order.items.add(order_item)
        return redirect("product-detail", pk = pk)