id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
36764	QuerySet.only() causes n+1 queries with reverse foreign key relation	Bernhard Vallant		"To reproduce I tried adding the following test to `tests.defer.tests.DeferTests`:


{{{#!python
def test_only_reverse_fk(self):
        qs = self.s1.primary_set.only(""name"")
        with self.assertNumQueries(1):
            for primary in qs:
                primary.name
}}}

This fails because it produces an additional query for every `Primary` instance to just fetch `related_id`:


{{{
test_only_reverse_fk (defer.tests.DeferTests.test_only_reverse_fk) failed:

    AssertionError('3 != 1 : 3 queries executed, 1 expected
    Captured  queries were:
        1. SELECT ""defer_primary"".""id"", ""defer_primary"".""name"" FROM ""defer_primary"" WHERE ""defer_primary"".""related_id"" = 1
        2. SELECT ""defer_primary"".""id"", ""defer_primary"".""related_id"" FROM ""defer_primary""  WHERE ""defer_primary"".""id"" = 1 LIMIT 21
        3. SELECT ""defer_primary"".""id"", ""defer_primary"".""related_id"" FROM ""defer_primary""  WHERE ""defer_primary"".""id"" = 2 LIMIT 21
    ')
}}}

When replacing the queryset with `qs = Primary.objects.filter(related_id=self.s1.pk).only(""name"")` basically the same SQL is produced, but no additional queries.
I think the additional queries are quite an unexpected behaviour and it is unnecessary to retrieve `related_id` for every row. Somehow this is also a dangerous behaviour as it can cause n + 1 queries when actually trying to optimize your query.

If this behaviour should be the expected one it should at least be documented and tested somewhere (I don't think there are tests for `only()` when using relations like this somewhere).
"	Bug	closed	Database layer (models, ORM)	dev	Normal	duplicate	queryset, only, defered		Unreviewed	0	0	0	0	0	0