django聚合:求和再求平均值 [英] django aggregation: sum then average

问题描述

使用Django的ORM annotate()和/或aggregate():我想基于一个类别字段进行汇总，然后对每个日期的类别值进行平均.我尝试使用两个annotate()语句来执行此操作，但遇到了FieldError.

Using django's ORM annotate() and/or aggregate(): I want to sum up based on one category field and then average over the category values per date. I tried to do it using two annotate() statements but got a FieldError.

我正在这样做:

queryset1 = self.data.values('date', 'category').annotate(sum_for_field=Sum('category'))

哪个输出的ValuesQuerySet对象具有以下内容(因此，每个类别值的总和):

Which outputs a ValuesQuerySet object with things like (so a sum for each value of category):

[{'category': 'apples', 'date': '2015-10-12', sum_for_field=2000},
 {'category': 'carrots', 'date': '2015-10-12', sum_for_field=5000},
 {'category': 'apples', 'date': '2015-10-13', sum_for_field=3000},
 {'category': 'carrots', 'date': '2015-10-13', sum_for_field=6000}, ...
]

然后，我想对每个日期的sum_for_field字段求平均值，以输出如下内容:

I then want to average the sum_for_field field for each date to output something like:

[ {'date': '2015-10-12', avg_final: 3500},
{'date': '2015-10-13', avg_final: 4500}, ...
]

我尝试这样做:

queryset2 = queryset1.values('date', 'sum_for_field')
result = queryset2.annotate(avg_final=Avg('sum_for_field'))

但是我收到了这个FieldError:

But I got this FieldError:

FieldError: FieldError: Cannot compute Avg('sum_for_field'): 'sum_for_field' is an aggregate

推荐答案

许多按组的聚合注释中的按组聚合注释通常是一个复杂的问题，但是Sum 中的 Avg是一个很复杂的问题更容易的特殊情况.

Aggregate annotation by group from many aggregate annotations by group is generally a complicated question, but Avg from Sum is a special much easier case.

表达式Avg('sum_for_field')评估为Sum('sum_for_field') / Count('category', distinct=True)，可以通过 Aggregate()表达式. Sum('sum_for_field')等于Sum('amount').

Expression Avg('sum_for_field') can be evaluated as Sum('sum_for_field') / Count('category', distinct=True) that can be evaluated by Aggregate() expressions. The Sum('sum_for_field') equals Sum('amount').

解决方案 :(预期名称:模型为Data，其字段为date，category，amount.)

Solution: (Expected names: The model is Data that has fields date, category, amount.)

qs = Data.objects.values('date').annotate(
    avg_final=Sum('amount') / Count('category', distinct=True)
)

(我确信，即使使用Subquery类，也不需要使用奇怪的extra()方法，也不需要原始SQL，当前的Django 1.11都无法解决非常相似的问题)

(I'm convinced that very similar questions would be without any solution by current Django 1.11, even with Subquery class, without using a strange extra() method and without raw SQL)

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