如何在购物车中正确使用 Django ORM Count

Question

我有这样的模型结构，我需要获取有关订购了多少比萨饼的信息。 我做了这样的事情PizzaOrder.objects.all().values('pizza').annotate(total=Count('pizza'))。它适用于PizzaOrder 中的count 等于1 的订单，但如果count 大于1，则显示错误的计数。所以我想以某种方式将 Django Count 与我的字段 count 连接起来，以便知道订购了多少比萨饼。

models.py

class Pizza(models.Model):
    name = models.CharField(max_length=256)


class Order(models.Model):
    order_number = models.IntegerField(primary_key=True)
    pizzas = models.ManyToManyField(to='Pizza', through='PizzaOrder')


class PizzaOrder(models.Model):
    pizza = models.ForeignKey(
        to=Pizza,
        on_delete=models.PROTECT,
    )
    order = models.ForeignKey(
        to=Order,
        on_delete=models.PROTECT,
    )
    count = models.SmallIntegerField()

Answer 1

你需要使用Sum函数而不是Count：

# annotating on the orders:
orders = Order.objects.annotate(total=Sum('pizzaorder__count'))

for order in orders:
    print(order.order_number, order.total)


# annotating on the pizzas:
pizzas = Pizza.objects.annotate(total=Sum('pizzaorder__count'))

for pizza in pizzas:
    print(pizza.name, pizza.total)


# aggregating irrespective of order / pizza:
pizza_count = PizzaOrder.objects.aggregate(total=Sum('count'))['total']

print(pizza_count)