How can I update my Django model using signals? - django

Question

How can I keep adding to the total price, when a new instance is created in the CartItem model and also deduct the price when an item is deleted ? , at the moment my current signals is only showing the price of the current product that I update or create.
signals.py
#receiver([post_save, post_delete], sender=CartItem, dispatch_uid="update_total")
def add_cart_receiver(sender, instance, *args, **kwargs):
total = 0
product_price = instance.item.price
quantity = instance.quantity
updated_total = Decimal(product_price) * int(quantity)
total += updated_total
if instance.items_cart.subtotal != total:
instance.items_cart.subtotal = total
instance.items_cart.save()
models.py
class CartItem(models.Model):
item = models.ForeignKey(Product, on_delete=models.CASCADE, blank=True, null=True)
items_cart = models.ForeignKey('Cart', blank=True, on_delete=models.CASCADE, null=True)
quantity = models.IntegerField(null=True, blank=True)
updated = models.DateTimeField(auto_now=True)
timestamp = models.DateTimeField(auto_now_add=True)
def __str__(self):
return 'item:{} cart:{} quantity:{}'.format(self.item, self.items_cart, self.quantity)
class Cart(models.Model):
user = models.ForeignKey(User, null=True, blank=True, on_delete=models.CASCADE)
subtotal = models.DecimalField(default=0.00, max_digits=100, decimal_places=2)
total = models.DecimalField(default=0.00, max_digits=100, decimal_places=2)
updated = models.DateTimeField(auto_now=True)
timestamp = models.DateTimeField(auto_now_add=True, null=True)
def __str__(self):
return 'id:{} user:{} subtotal:{} total:{} updated:{}'.format(self.id, self.user, self.subtotal, self.total, self.updated)
views.py
def cart_add(request):
product_id = request.POST.get('product_id')
try:
product_obj = Product.objects.get(id=product_id)
quantity = request.POST.get('quantity_field')
cart_obj, new_obj = Cart.objects.new_or_get(request)
last_qty = CartItem.objects.filter(item=product_obj, items_cart=cart_obj).reverse()[0].delete()
new_qty = CartItem(item=product_obj, items_cart=cart_obj, quantity=quantity)
new_qty.save()
return redirect("cart:home")
except ValueError:
print("Unable to increase quantity")
return redirect("cart:home")

Answer 1

I figured out the problem I was having, below is my solution, to keep accumulating the total price of all items in my current cart, I have done this by looping over a Queryset of 'cart_items' and calculating the price * quantity, then adding it to a counter ('total'), then saving it to my 'Cart' model field 'total'. The receiver is collecting 'post_save' and 'post_delete' signals. Hopefully this will help someone else.
signals.py
#receiver([post_save,post_delete], sender=CartItem, dispatch_uid="update_total")
def add_cart_receiver(sender, instance, **kwargs):
cart = instance.items_cart.id
cart_items = CartItem.objects.filter(items_cart=cart)
cart_total = 0
for element in cart_items:
cart_total += (Decimal(element.item.price) * int(element.quantity))
instance.items_cart.subtotal = cart_total
instance.items_cart.save()