django - 使用案例进行注释并按以下方式在组中聚合注释时 ORM 代码中的 KeyError

Question

我们在一个相当复杂的项目中遇到了这个问题，但我设法在一个虚拟项目中重现了它:

这是 python3.6 和 postgres10 上的 django 1.11.7:

模型.py:

from django.db import models


class Thing(models.Model):

    multiplierA = models.IntegerField()
    multiplierB = models.IntegerField()


class Data(models.Model):

    thing = models.ForeignKey('Thing')
    multiplier_choice = models.CharField(max_length=1, choices=(('A', 'use multiplier A'), ('B', 'use multiplier B')))
    option = models.IntegerField(choices=((1, 'option 1'), (2, 'option 2')))
    percentage = models.FloatField()

测试.py:

from django.db.models import Case, F, FloatField, IntegerField, Sum, When
from django.test import TestCase

from .models import Data, Thing


class AnnotateTests(TestCase):

    def test_simple(self):

        thing = Thing.objects.create(multiplierA=2, multiplierB=3)

        Data.objects.create(thing=thing, multiplier_choice='A', option=1, percentage=0.2)
        Data.objects.create(thing=thing, multiplier_choice='A', option=2, percentage=0.3)
        Data.objects.create(thing=thing, multiplier_choice='A', option=3, percentage=0.1)

        Data.objects.create(thing=thing, multiplier_choice='B', option=1, percentage=0.1)
        Data.objects.create(thing=thing, multiplier_choice='B', option=2, percentage=0.4)
        Data.objects.create(thing=thing, multiplier_choice='B', option=3, percentage=0.5)


        whens = [
            When(multiplier_choice='A', then=F('thing__multiplierA')),
            When(multiplier_choice='B', then=F('thing__multiplierB'))
        ]

        multiplier_case = Case(*whens, output_field=IntegerField(), default=0)


        qs = (Data.objects
              # select only certain options to sum up for each thing:
              .filter(thing=thing, option__in=[1, 2])
              # select the correct multiplier
              .annotate(multiplier=multiplier_case)
              # group by thing => sum of percentage * multiplier
              .values('thing')
              .annotate(amount_sum=Sum(F('percentage') * F('multiplier')))
              .values('amount_sum'))

        print(qs.values('thing__id', 'amount_sum'))

运行此测试将导致以下回溯:

======================================================================
ERROR: test_simple (annotate.tests.AnnotateTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/robin/src/ormweirdness/annotate/tests.py", line 34, in test_simple
    .annotate(amount_sum=Sum(F('percentage') * F('multiplier')))
  File "/Users/robin/.virtualenvs/ormweirdness/lib/python3.6/site-packages/django/db/models/query.py", line 945, in annotate
    clone.query.add_annotation(annotation, alias, is_summary=False)
  File "/Users/robin/.virtualenvs/ormweirdness/lib/python3.6/site-packages/django/db/models/sql/query.py", line 973, in add_annotation
    summarize=is_summary)
  File "/Users/robin/.virtualenvs/ormweirdness/lib/python3.6/site-packages/django/db/models/aggregates.py", line 19, in resolve_expression
    c = super(Aggregate, self).resolve_expression(query, allow_joins, reuse, summarize)
  File "/Users/robin/.virtualenvs/ormweirdness/lib/python3.6/site-packages/django/db/models/expressions.py", line 548, in resolve_expression
    c.source_expressions[pos] = arg.resolve_expression(query, allow_joins, reuse, summarize, for_save)
  File "/Users/robin/.virtualenvs/ormweirdness/lib/python3.6/site-packages/django/db/models/expressions.py", line 412, in resolve_expression
    c.rhs = c.rhs.resolve_expression(query, allow_joins, reuse, summarize, for_save)
  File "/Users/robin/.virtualenvs/ormweirdness/lib/python3.6/site-packages/django/db/models/expressions.py", line 471, in resolve_expression
    return query.resolve_ref(self.name, allow_joins, reuse, summarize)
  File "/Users/robin/.virtualenvs/ormweirdness/lib/python3.6/site-packages/django/db/models/sql/query.py", line 1472, in resolve_ref
    return self.annotation_select[name]
KeyError: 'multiplier'

----------------------------------------------------------------------

我在 django-users mailing list 上发现有人似乎也有这个问题.很遗憾，没有回复。

这是怎么回事？

Answer 1

尽管我仍然认为上述方法应该有效(ORM 中的错误？)，但我发现将乘法移动到 Case 语句并使用 Case直接在 Sum 中声明可以解决这个问题，因为现在 Sum 不必进行注释查找:

whens = [
    When(multiplier_choice='A', then=F('thing__multiplierA') * F('percentage')),
    When(multiplier_choice='B', then=F('thing__multiplierB') * F('percentage'))
]

multiplier_case = Case(*whens, output_field=FloatField(), default=0)


data_qs = (Data.objects
           # select only certain options to sum up for each thing:
           .filter(thing=thing, option__in=[1, 2])
           .values('thing')
           # group by thing => sum of percentage * multiplier
           .annotate(amount_sum=Sum(multiplier_case))
           .values('amount_sum')
           .order_by('thing'))

生成以下 SQL:

SELECT SUM(CASE
               WHEN "annotate_data"."multiplier_choice" = A THEN ("annotate_thing"."multiplierA" * "annotate_data"."percentage")
               WHEN "annotate_data"."multiplier_choice" = B THEN ("annotate_thing"."multiplierB" * "annotate_data"."percentage")
               ELSE 0
           END) AS "amount_sum"
FROM "annotate_data"
INNER JOIN "annotate_thing" ON ("annotate_data"."thing_id" = "annotate_thing"."id")
WHERE ("annotate_data"."thing_id" = 1
       AND "annotate_data"."option" IN (1,
                                        2))
GROUP BY "annotate_data"."thing_id"
ORDER BY "annotate_data"."thing_id" ASC

如果有人找到了一个解决方案，该解决方案并不意味着在每个 When 中重复整个计算(这可能比我们正在做的复杂得多)，我会接受你的答案解决方案:)