计算Django中模型属性的总和 [英] Calculate the sum of model properties in Django

问题描述

我有一个模型 Order ，该模型具有一个根据计算 order_total 的属性通过外键链接的OrderItem 。

I have a model Order which has a property that calculates an order_total based on OrderItems linked by foreign key.

我想计算 Order 实例的 order_total 个属性。

I would like to calculate the sum of a number of Order instances' order_total properties.

有没有办法做到这一点？

Is there a way of doing this?

class Order(models.Model):
    customer = models.ForeignKey(Customer)
    placed = models.DateField()

    ...

    def get_total_cost(self):
        return sum(item.get_cost() for item in self.items.all())

    order_total = property(get_total_cost)


class OrderItem(models.Model):
    order = models.ForeignKey(Order, related_name="items")
    product = models.ForeignKey(Product, related_name="order_items")
    quantity = models.PositiveIntegerField(default=1)

    ...

    def get_cost(self):
        return self.product.price * self.quantity

这是我的查询：

>>> Order.objects.all().aggregate(Sum("order_total"))

哪个会返回此错误：

django.core.exceptions.FieldError: Cannot resolve keyword 'order_total' into field. Choices are: placed, customer, customer_id, id, items, paid

推荐答案

您需要使用双下划线 __ 从 OrderItem 模型中以 order 作为查找字段的order_total的外键查找：

You need to use double underscore __ for foreign key lookup of order_total from OrderItem model with order as the lookup field:

odrerObj = OrderItem.objects.all().aggregate(sum_order = Sum("order__order_total"))

但是如果您需要 Order oject的order_total属性的总和，则可以使用以下内容：

But if you need sum of Order oject's order_total property, you can use something like this :

order_list = Order.objects.all()
sum = 0
for item in order_list :
    sum = sum + item.order_total()

print sum

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