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
27002	Redundant database query rendering a ModelChoiceField with RadioSelect or CheckboxSelectMultiple	Alex Hill	nobody	"Followup to #27001:

Rendering a ModelChoiceField with RadioSelect and CheckboxSelectMultiple, there's a redundant iteration that triggers an unnecessary database call.

Here's the relevant code from ForNode.render ([https://github.com/django/django/blob/ccd5a23fba91e66656bb19b60bb6d737cab728f5/django/template/defaulttags.py#L164 GitHub link]):

{{{#!python
            if not hasattr(values, '__len__'):  # line 164
                values = list(values)
            len_values = len(values)
            ...
            for i, item in enumerate(values):  # line 177
                ...
}}}

In this code, `values` is a BoundField. BoundField defines `__len__` like this:

{{{#!python
    def __len__(self):
        return len(list(self.__iter__()))
}}}

So `len(values)` on line 164 triggers an iteration over `values` (and thus a database query), then the for loop at line 177 triggers another.

The options as I see them are
1. Remove `BoundField.__len__` so that `list(values)` is called in `ForNode.render`. This fixes the problem without breaking any tests. It is backwards-incompatible (e.g. with an existing template passing a boundfield to the `length` template filter), but not with anything documented.
2. Cache iterator results somewhere internally so that the actual query is only executed once. This is easy to do on BoundField with a cached property. No backward compatibility concerns here.
3. Unconditionally call `list(values)` in ForNode. Also works, but a bit heavy-handed and not necessary if one of the first two options are acceptable.

I think (2) is the best choice. It's pretty clean, and it means the fix can be implemented in 1.10 (and 1.8 LTS?) as a bug fix without backward compatibility concerns."	Cleanup/optimization	closed	Forms	dev	Normal	fixed			Accepted	1	0	0	0	0	0