Coalesce function should work with subqueries

[Tzu-ping Chung]

Say I have a model

class Foo(models.Model):
    val = models.IntegerField()

with data 1, 2, 3, 4, 5. In SQL I can do (roughly)

SELECT val FROM foo WHERE val >= COALESCE((SELECT val FROM foo WHERE val=6 LIMIT 1 OFFSET 0), 3);

to get 3, 4, and 5. Translated into Django ORM query language:

Foo.objects.filter(val__gte=Coalesce(Foo.objects.filter(val=6).values_list('val', flat=True)[:1], 3))

I get a ProgrammingError because Django does not handle subqueries, and passes the QuerySet object directly to the SQL backend.

The reason I don’t want to use first() instead is that I am doing some query-joining in a loop, and repeated first() calls result in lots of queries, dragging the database down.

[Ekin Dursun]

Thanks to Subquery, I think this ticket can be closed.

[Simon Charette]

Confirmed support even without the Subquery wrapping in ​this PR.

[Carlton Gibson <carlton@…>]

In b30c0081:

Refs #27412 -- Confirmed support for executing Coalesce(subquery).

This has been supported for subqueries wrapped in Subquery since the expression
was introduced and for Queryset directly since Subquery resolves to sql.Query.

Piggy-backed on the existing tests covering Coalesce handling of EmptyResultSet
as it seemed like a proper location to combine testing.