[Django] #31606: Cannot mix Exists expression with keyword arguments to When

[Django]

#31606: Cannot mix Exists expression with keyword arguments to When
------------------------------------------------+------------------------
               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>
Django <https://code.djangoproject.com/>
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