如何汇总基于两列的计算平均值? [英] How to aggregate the average of a calculation based on two columns?
问题描述
我想编写一个Django查询来给我表中所有行的平均值.我的模特看起来像
I want to write a Django query to give me the average across all rows in my table. My model looks like
class StatByDow(models.Model):
total_score = models.DecimalField(default=0, max_digits=12, decimal_places=2)
num_articles = models.IntegerField(default=0)
day_of_week = IntegerField(
null=True,
validators=[
MaxValueValidator(6),
MinValueValidator(0)
]
)
然后我尝试像这样计算平均值
and I attempt to calculate the average like this
everything_avg = StatByDow.objects.all().aggregate(Avg(Func(F('total_score') / F('num_articles'))))
但这会导致错误
File "/Users/davea/Documents/workspace/mainsite_project/venv/lib/python3.7/site-packages/django/db/models/query.py", line 362, in aggregate
raise TypeError("Complex aggregates require an alias")
TypeError: Complex aggregates require an alias
计算平均值的正确方法是什么?
What's the right way to calculate the average?
推荐答案
您不需要Func
进行划分,但需要协调两种不同的字段类型.使用 ExpressionWrapper 在Avg
:
You don't need Func
for the division, but you need to reconcile the two different field types. Use an ExpressionWrapper around Avg
:
from django.db.models import ExpressionWrapper
everything_avg = (StatByDow.objects
.aggregate(avg=ExpressionWrapper(
Avg(F('total_score') / F('num_articles')),
DecimalField()
))
)
您还可以使用从整数到小数的Cast
(不适用于PostgreSQL,该对象遵循Django的语法::numeric(NONE, NONE)
)或围绕除法的ExpressionWrapper
,但是最后一个ExpressionWrapper
最快解决方案,因为它最后只发生一次.
You could also use a Cast
from integer to decimal (not with PostgreSQL, which objects to Django's syntax ::numeric(NONE, NONE)
) or an ExpressionWrapper
around the division, but just one ExpressionWrapper
at the end is the quickest solution as it happens once at the end.
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