我在Django ORM中遇到了一个有线的情况。它返回了一个错误的计算和意外地乘以我不想要的条目数,它的行为完全是有线的。
这些是我的模型
class Person(models.Model):
name = models.CharField(max_length=100)
class Purchase(models.Model):
person = models.ForeignKey(
Person,
on_delete=models.CASCADE,
related_name='person_purchase'
)
amount = models.DecimalField(decimal_places=2, max_digits=5)
class Consumption(models.Model):
person = models.ForeignKey(
Person,
on_delete=models.CASCADE,
related_name='person_consumption'
)
amount = models.DecimalField(decimal_places=2, max_digits=5)
这是我的疑问。
person_wise_payable = Person.objects.annotate(
difference=ExpressionWrapper(
Coalesce(Sum('person_purchase__amount'), Value(0)) - Coalesce(Sum('person_consumption__amount'), Value(0)),
output_field=FloatField()
),
)
for i in person_wise_payable:
print(i.difference, i.name)
我想知道人的购买和消费之间的区别。
例如,我有3个人。foo, doe, jhon
而这些都是他们的作品
Purchase models entries:
foo = 5
doe = 2
doe = 3
doe = 3
Consumption models entries:
foo = 1
foo = 2
foo = 2
doe = 1
doe = 1
jhon = 2
所以你看上面。
foo total purchase is 5
doe total purchase is 8
jhon total purchase is 0 (coz, no entry of him)
和
foo total consumption is 5
doe total consumption is 2
jhon total consumption is 2
所以,如果我们减去预期的产出差额。consumption
从 purchase
foo : 5 - 5 = 0
doe: 8 - 2 = 6
jhon: 0 - 2 = -2
我希望你能明白我想做什么,预期的输出是什么。
foo 0, doe 6 and jhon -2
但问题是,我现在的查询没有返回上面这样的输出,而是返回非常非常多的有线,见下面的有线结果。
-2.0 jhon
10.0 doe
10.0 foo
谁能帮帮我,我怎么才能把它做对?我这几天都在努力,还不能实现这个功能
如果你做了多个JOINs,那么它们就像一个乘数,因为你做了一个查询。
SELECT SUM(purchase.amount) - SUM(consumption.amount)
FROM person
LEFT OUTER JOIN purchase
LEFT OUTER JOIN consumption
于是 一样 purchase.amount
重复的次数多了,因为有相关的 consumption
s,以及 一样 consumption.amount
重复的次数多了,因为有相关的 purchase
s.
你可以用子查询来解决这个问题,例如用。
person_wise_payable = Person.objects.annotate(
difference=Coalesce(Subquery(
Purchase.objects.filter(
person=OuterRef('pk')
).values('person').annotate(
sum=Sum('amount')
).values('sum')[:1]
), Value(0)) - Coalesce(Subquery(
Consumption.objects.filter(
person=OuterRef('pk')
).values('person').annotate(
sum=Sum('amount')
).values('sum')[:1]
), Value(0))
)