Using annotate and extra in ORM

[Zerq]

In simple example:

class Foo(models.Model):
     pass
class Bar(models.Model):
     baz = models.ForeignKey('Foo', on_delete=models.PROTECT)

I want to count reverse relation count and then select_for_update:

Foo.objects.annotate(Count('bar')).select_for_update().get(...)

but this does not work: django.db.utils.NotSupportedError: FOR UPDATE is not allowed with GROUP BY clause

To circumvent this problem I used:

Foo.extra(select={'bar_count': 'SELECT COUNT(*) FROM "app_bar" U0 WHERE U0."baz_id" = "app_foo"."id"'}).select_for_update().get(...)

I file this issue to show usage of extra, as asked in documentation.