Django-按组注释加权AVG [英] Django - Annotating Weighted AVG by Group

问题描述

我有下面的模型管理器，可以对SalesRecord进行操作：

I've got the following model manager that operates on a SalesRecord:

def by_variety_and_date(self, start_date, end_date):
    return self.model.objects.filter(
        date__range=(start_date, end_date)
    ).values(
        "variety"
    ).annotate(
        qty_applied=Sum('qty_applied'),
        margin=Avg('margin')
    )

我真正想要的是 margin = Avg（'margin'）以返回加权平均值，基于 qty_applied 。有没有办法使用Django的注释/汇总查询来做到这一点？我一直在尝试将.aggregate（）插入字符串的结尾，但是我仍然希望按照此查询集的描述，每个品种的平均值。

What I'd really like is for the margin=Avg('margin') to return a weighted average, based on qty_applied. Is there a way to do this with Django's annotate/aggregate queries? I've been experimenting with stringing .aggregate() to the end of this, but I still want the average per variety as described by this queryset.

这种情况下的模型如下：

The model in this case looks like this:

class Sale(models.Model):
    margin = models.DecimalField(null=True, blank=True, decimal_places=2, max_digits=12)
    qty_applied = models.IntegerField(null=True, blank=True)
    variety = models.ForeignKey(Variety, null=True)
    totals = Totals()

EDIT

这是我最终的结果。有点古怪，但是可以解决问题。

This is what I finally ended up with. It's a bit wonky, but it does the trick.

def by_variety_and_date(self, start_date, end_date):
    return self.model.objects.filter(
        date__range=(start_date, end_date)
    ).values(
        "variety"
    ).annotate(
        qty_applied=Sum('qty_applied'),
        profit=Sum('profit'),
        cogs=Sum('cogs'),
        sales=Sum('value'),
        margin=Sum(
            (F('qty_applied')*F('margin')), output_field=FloatField()
        ) / Sum(
            'qty_applied', output_field=FloatField())
    )

我将F对象用作 @WTower 建议将每个对象的 margin 乘以它的 qty_applied ，然后将整个内容包装在<$ c中$ c> Sum 并将其除以其中所有对象的 qty_applied 的 Sum 群组。奇迹般有效！

I used the F objects as @WTower suggested to multiply each object's margin by it's qty_applied, then wrapped the whole thing in a Sum and divided it by the Sum of the qty_applied of all objects in the group. Works like a charm!

推荐答案

您可以使用 F（）表达式

You can use the F() expression

>>> from django.db.models import F, FloatField, Sum
>>> MyModel.objects.all().aggregate(
...    w_avg=Sum((F('a')*F('b'))/F('b'), output_field=FloatField()))

有关聚合的更多信息此处。

或者，您可以使用自定义SQL（不建议）或管理器中的自定义方法。

Alternatively, you can use custom SQL (not recommended) or a custom method in the manager.

这篇关于Django-按组注释加权AVG的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！