Subquery.as_sql() generates invalid SQL.

[M1ha Shvn]

Since ​this commit Subquery.as_sql(...) method returns incorrect SQL removing first and last symbols instead of absent breakets. Adding Subquery().query.subquery = True attribute fixes the problem. From my point of view, it should be set in Subquery constructor.

from django.db import connection
from apps.models import App

q = Subquery(App.objects.all())

print(str(q.query))

# Output SQL is valid:
#  'SELECT "apps_app"."id", "apps_app"."name" FROM "apps_app"'

print(q.as_sql(q.query.get_compiler('default'), connection))

# Outptut SQL is invalid (no S letter at the beggining and " symbol at the end):
# ('(ELECT "apps_app"."id", "apps_app"."name" FROM "apps_app)', ())

q.query.subquery = True
print(q.as_sql(q.query.get_compiler('default'), connection))

# Outputs correct result
('(SELECT "apps_app"."id", "apps_app"."name" FROM "apps_app")', ())

[Mariusz Felisiak]

Sounds reasonable.

[Simon Charette]

Sounds reasonable to me as well, I'd only suggest we .clone() the query before altering though.

[Mohamed Nabil Rady]

Pull request: ​https://github.com/django/django/pull/15320.

Ran back-end tests and they went fine.

[Mariusz Felisiak <felisiak.mariusz@…>]

In f37face3:

Fixed #33435 -- Fixed invalid SQL generatered by Subquery.as_sql().