#34858: Bug in output field handling
               Reporter:  Toan Vuong     |          Owner:  nobody
                   Type:  Uncategorized  |         Status:  new
              Component:  Uncategorized  |        Version:  4.2
               Severity:  Normal         |       Keywords:
           Triage Stage:  Unreviewed     |      Has patch:  0
    Needs documentation:  0              |    Needs tests:  0
Patch needs improvement:  0              |  Easy pickings:  0
                  UI/UX:  0              |
 Tested on Django 4.2.4.

 My model is defined like this:

 class Choice(models.Model):
     choice_text = models.CharField(max_length=200)
     votes = models.IntegerField(default=0)
     arbitrary_num = models.PositiveIntegerField(default=1)
     arbitrary_num2 = models.PositiveIntegerField(default=2)

 And this query is failing with Oracle (Tested on 19.3), but not Postgres:

 s = Coalesce(F('arbitrary_num') + F('arbitrary_num2'),
                  Value(0, PositiveIntegerField()))
 case = Case(
         When(arbitrary_num=1, then=Value(1, PositiveIntegerField())),

 qs = Choice.objects.annotate(ss=case)

 #qs = Choice.objects.annotate(s=s)


 The error (It's talking about the `Coalesce` function not having an

 django.core.exceptions.FieldError: Expression contains mixed types:
 IntegerField, PositiveIntegerField. You must set output_field.

 I believe the error is correct, because I think adding two
 `PositiveIntegerField()` seem to result in an `IntegerField`, so we *need*
 to specify an `output_field` on `Coalesce` to tell Django the proper
 output type to use. My question is that this seem to only throw an error
 in Oracle, and *not* Postgres. So it seems like the behavior on Postgres
 is unexpected? Somehow, the `Case` statement swallows the exception in
 Postgres and generates correct result. This can be demonstrated by the
 queryset where the `Coalesce` function is annotated directly without a
 case statement -- in this case, both Postgres and Oracle fails with the
 above error.

 Note that this used to work prior to 4.2 (we upgraded from 3.x, where this
 used to work), although again I believe it was incorrect behavior and an
 `output_field` *should* be specified.

Ticket URL: <https://code.djangoproject.com/ticket/34858>
Django <https://code.djangoproject.com/>
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