QuerySet.filter() with a sliced QuerySet crashes

[Sergey Fedoseev]

PersonSkill.objects.filter(skill=Skill.objects.all()[:1])

ProgrammingError: ОШИБКА:  подзапрос должен вернуть только один столбец (subquery should return single column)
LINE 1: ...nskill" WHERE "test_app_personskill"."skill_id" = (SELECT "t...

Introduced in ec50937bcbe160e658ef881021402e156beb0eaf.

[Simon Charette]

We should try to error out with a more appropriate message but I don't think we ever officially supported passing a queryset to an __exact lookup. I vaguely remember a ticket about it but I can't find it anymore.

I think you should be using an __in lookup instead as your query will fail with more than one row returned by a subquery used as an expression as soon as the table backing up your Skill model has more than two rows.

[Tim Graham]

Maybe #25284 is the related ticket Simon thought of.

[Simon Charette]

That's the ticket I was referring to, thanks for looking that up Tim!

[Tim Graham]

Here's a regression test. It's not clear to me why this shouldn't be allowed. What should the error message say?

tests/lookup/tests.py

@@ -138 +138 @@
         with self.assertNumQueries(expected_num_queries):
             self.assertEqual(Author.objects.in_bulk(authors.keys()), authors)
 
+    def test_filter_with_sliced(self):
+         self.assertSequenceEqual(
+            Article.objects.filter(author=Author.objects.all()[:1]),
+            [self.a4, self.a2, self.a3, self.a1],
+        )
+
     def test_values(self):
         # values() returns a list of dictionaries instead of object instances --
         # and you can specify which fields you want to retrieve.

[Simon Charette]

What should the error message say?

That __in should be used with multiple elements or a single model instance should be passed? Maybe we could fix the issue and officialize and support as well.

Any thoughts Sergey?

[Sergey Fedoseev]

Replying to Tim Graham:

It's not clear to me why this shouldn't be allowed.

+1

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In 1b73ccc4:

Fixed #28497 -- Restored the ability to use sliced QuerySets with exact.

Regression in ec50937bcbe160e658ef881021402e156beb0eaf.

Thanks Simon Charette for review.

[Tim Graham <timograham@…>]

In 9b3b7804:

[2.0.x] Fixed #28497 -- Restored the ability to use sliced QuerySets with exact.

Regression in ec50937bcbe160e658ef881021402e156beb0eaf.

Thanks Simon Charette for review.

Backport of 1b73ccc4bf78af905f72f4658cf463f38ebf7b97 from master