def update(self, product, quantity, unit_price=None):
		try:
			item = models.Item.objects.get(
				cart=self.cart,
				product=product,
			)
		except models.Item.DoesNotExist:
			raise ItemDoesNotExist
		else:
			item.quantity = quantity
			item.save()

def update_cart(request, product_id, quantity):
	product = Product.objects.get(id=product_id)
	cart = Cart(request)
	cart.update(product, quantity, product.price)

def update_cart(request, product_id, quantity):
	product = Product.objects.get(id=product_id)
	cart = Cart(request)
	cart.remove(product)
	cart.add(product, product.price, quantity)