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
29159	ModelChoiceIterator triggers duplicate queries when choices are cast to list	François Freitag	François Freitag	"When there are no `prefetch_related()` on `ModelChoiceIterator`'s queryset, it attempts to use the `iterator()` method to avoid loading all choices in memory. ([https://docs.djangoproject.com/en/2.0/ref/models/querysets/#prefetch-related because iterator() and prefetch_related() don't make sense together.])

That results in a duplicate query if the choices have been evaluated before, because calling `iterator()` will clone the `QuerySet`, resetting its result cache in the process.

{{{
class TestModelChoiceField(TestCase):  
    def test_queryset_result_cache_is_reused(self):
        choice = ChoiceOptionModel.objects.create(name=""choice 1"")
        f = ModelChoiceField(ChoiceOptionModel.objects.all())
        with self.assertNumQueries(1):
            self.assertEqual(
                # list calls both __len__ and __iter__
                list(f.choices),
                [('', '---------'), (choice.pk, str(choice))],
            )
}}}


Fails with:
{{{
FAIL: test_queryset_result_cache_is_reused (forms_tests.test_tmp.TestModelChoiceField)                                                                         
----------------------------------------------------------------------                                                                                         
Traceback (most recent call last):                                                                                                                             
  File ""django/tests/forms_tests/test_tmp.py"", line 19, in test_queryset_result_cache_is_reused
    [('', '---------'), (choice.pk, str(choice))],
  File ""django/django/test/testcases.py"", line 80, in __exit__
    '%d. %s' % (i, query['sql']) for i, query in enumerate(self.captured_queries, start=1)
AssertionError: 2 != 1 : 2 queries executed, 1 expected
Captured queries were:
1. SELECT ""forms_tests_choiceoptionmodel"".""id"", ""forms_tests_choiceoptionmodel"".""name"" FROM ""forms_tests_choiceoptionmodel"" ORDER BY ""forms_tests_choiceoptionmodel"".""name"" ASC
2. SELECT ""forms_tests_choiceoptionmodel"".""id"", ""forms_tests_choiceoptionmodel"".""name"" FROM ""forms_tests_choiceoptionmodel"" ORDER BY ""forms_tests_choiceoption$odel"".""name"" ASC
}}}"	Cleanup/optimization	closed	Forms	1.8	Normal	fixed			Ready for checkin	1	0	0	0	0	0