#31606: Cannot mix Exists expression with keyword arguments to When
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Reporter: rheard | Owner: nobody
Type: Cleanup/optimization | Status: new
Component: Uncategorized | Version: 3.0
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 1
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 1
UI/UX: 0 |
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I don't seem to be able to provide an Exists expression to When alongside
keyword arguments like you can with filter. For instance, consider:
{{{
class State(models.Model):
pass
class County(models.Model):
name = CharField(max_length=50)
state = ForeignKey(State, related_name='counties')
}}}
I can execute the following query just fine:
{{{
County.objects.filter(
Exists(State.objects.filter(counties=OuterRef('pk'), name="Texas")),
name="Dallas",
)
}}}
But a similar query using When does not work:
{{{
>>> County.objects.annotate(
status=Case(
When(Exists(State.objects.filter(counties=OuterRef('pk'),
name="Texas")), name="Dallas", then=Value("DALLAS COUNTY")),
default=Value("ELSEWHERE"),
))
}}}
Instead the arguments must be wrapped in a Q object:
{{{
>>> County.objects.annotate(
status=Case(
When(Q(Exists(State.objects.filter(counties=OuterRef('pk'),
name="Texas")), name="Dallas"), then=Value("DALLAS COUNTY")),
default=Value("ELSEWHERE"),
))
}}}
This is inconvenient and inconsistent with how filter works, as shown.
When's __init__ method can be modified to allow similar input as filter.
[https://github.com/rheard/django/commit/978e796bb18414d995d515ab7923831149300977
Code is in a branch in my repo], but as this is my first time contributing
to Django, I want to make sure I open a ticket and get feedback first.
--
Ticket URL: <https://code.djangoproject.com/ticket/31606>
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