Queryset count does not work after union when annotations on aggregation are unused.

[Marcin]

While upgrading to Django 4 I've found a bug with count() which can be replicated using the following code. Notice that evaluating the queryset works fine, the issue is that for some reason, when counting django is not including total_count column from the second queryset in the query and this is causing union to fail. In Django 3 the code works fine and produces a correct SQL. I've attached an example project which can be used to reproduce the bug, with command tox -e ALL.

models.py

class Foo(models.Model):
    name = models.TextField()

class Bar(models.Model):
    foo = models.ForeignKey(Foo, on_delete=models.CASCADE)
    count = models.IntegerField()

tests.py

qs1 = Foo.objects.all().annotate(
    _count=models.Sum('bar__count')
).values('name').annotate(
    total_count=models.F('_count')
).values('name', 'total_count')

qs2 = Bar.objects.all().annotate(
    name=models.F('foo__name'),
    total_count=models.Value(0, output_field=models.IntegerField())
).values('name', 'total_count')

self.assertEqual(list(qs1.union(qs2, all=True)), [])
self.assertEqual(qs1.union(qs2, all=True).count(), 0)

Exception thrown in Django 4+

Traceback (most recent call last):
  File "/home/user/foo/bar/tests.py", line 21, in test_recreate_bug
    self.assertEqual(qs1.union(qs2, all=True).count(), 0)
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/models/query.py", line 618, in count
    return self.query.get_count(using=self.db)
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/models/sql/query.py", line 616, in get_count
    return obj.get_aggregation(using, {"__count": Count("*")})["__count"]
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/models/sql/query.py", line 602, in get_aggregation
    result = compiler.execute_sql(SINGLE)
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/models/sql/compiler.py", line 1562, in execute_sql
    cursor.execute(sql, params)
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/backends/utils.py", line 79, in execute
    return self._execute_with_wrappers(
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/backends/utils.py", line 92, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/backends/utils.py", line 100, in _execute 
    with self.db.wrap_database_errors:
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/utils.py", line 91, in __exit__ 
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/backends/utils.py", line 105, in _execute 
    return self.cursor.execute(sql, params)
  File "/home/user/foo/.tox/django5/lib/python3.10/site-packages/django/db/backends/sqlite3/base.py", line 328, in execute
    return super().execute(query, params)
django.db.utils.OperationalError: SELECTs to the left and right of UNION ALL do not have the same number of result columns

[Marcin]

minimal example

[Mariusz Felisiak]

Thanks for the report.

Regression in 59bea9efd2768102fc9d3aedda469502c218e9b7.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 7727892:

Fixed #35042 -- Fixed a count() crash on combined queries.

Regression in 59bea9efd2768102fc9d3aedda469502c218e9b7.

Thanks Marcin for the report.