Regression in query counts using RadioSelect with ModelChoiceField

[Alex Hill]

Before 1.9, I had on my list of things to look at the fact that a standard ModelChoiceField with a RadioSelect takes two queries. Now looking at this in 1.10, the number of queries has ballooned (to 11 queries in my simple test case).

I've not got far in looking for a solution, but this test passes in 1.9 and fails in 1.10 and master with AssertionError: 11 != 2 : 11 queries executed, 2 expected:

    def test_radioselect_num_queries(self):
        class CategoriesForm(forms.Form):
            categories = forms.ModelChoiceField(
                queryset=Category.objects.all(),
                widget=forms.RadioSelect
            )

        template = Template('{% for w in form.categories %}{{ w }}{% endfor %}')

        with self.assertNumQueries(2):
            template.render(Context({'form': CategoriesForm()}))

[Alex Hill]

Branch with failing test here: ​https://github.com/django/django/compare/master...AlexHill:ticket_27001

[Alex Hill]

Created a pull request with a fix.

ChoiceFieldRenderer doesn't have __iter__ defined, so Python iterates over it by calling __getitem__ with an increasing index until an exception is raised. ChoiceFieldRenderer.__getitem__ calls list on itself, which turns iteration into an n² operation. When the choices are backed by a queryset as in ModelChoiceField, that means lots of redundant database queries.

Fixed by adding an __iter__ method to ChoiceFieldRenderer and changing __getitem__ to use it, so that indexing still works.

[Simon Charette]

​PR

[Simon Charette]

As this is a regression in 1.10 I suppose this should be backported.

[Tim Graham <timograham@…>]

In c5ebfda:

Fixed #27001 -- Fixed a query count regression in ModelChoiceField with RadioSelect.

[Tim Graham <timograham@…>]

In 86ae2b2:

[1.10.x] Fixed #27001 -- Fixed a query count regression in ModelChoiceField with RadioSelect.

Backport of c5ebfda00226e3695cadbc13ea9ce4c5951d3ed0 from master