#25307: Cannot use .annotate with conditional expressions
-------------------------------------+-------------------------------------
Reporter: jproffitt | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by jproffitt):
That does make it break as well, but not simply just the use of distinct.
If you use EITHER `.distinct()` OR `.annotate()` along with an
`.aggregate()` that contains a conditional expression. (not sure exactly
what part of the condition expression, but it must be either because of
the `Case()` or the `When()`.
I have made a completely useless example using the polls app in the django
tutorial. I copied the polls models exactly, and created this query, which
will cause the error to happen:
{{{
counts = models.Choice.objects.annotate(
used_count=Count('question')
).distinct().aggregate(
yes=Sum(Case(
When(choice_text='yes', then=F('votes')),
default=0,
output_field=IntegerField()
)),
no=Sum(Case(
When(choice_text='no', then=F('votes')),
default=0,
output_field=IntegerField()
)),
)
}}}
(Obviously, then `used_count` will always be 1, and its pretty pointless
in this example, but you get the idea).
So if you remove the either the `.annotate()` OR the `.distinct()`, the
query still breaks the same way. The presence of either one causes it to
break. But if you remove BOTH, the query runs fine.
I do not know anything about the internals of the Query api, or I would
help fix the problem. But it seems there is a problem when combining
certain queryset methods like `annotate` or `distinct` with `aggregate`s
that have condition expressions.
I know it has to do with condition expressions, because doing the query
like the following works just fine:
{{{
counts = models.Choice.objects.annotate(
used_count=Count('question')
).distinct().aggregate(
yes=Sum('votes'),
)
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/25307#comment:12>
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