ModelChoiceFilter's queryset kwarg attached to memoized fields - which can cause issues

[shangxiao]

Given the following filterset (details omitted for brevity):

def get_bar(request):
    return Bar.objects.filter(…) if request else Bar.objects.none()
    
class FooFilterSet(FilterSet):
    bar = ModelChoiceFilter(queryset=get_bar)

If you happen to access base_filters at module level and (or anywhere without a request) like so:

FooFilterSet.base_filters['bar'].field

Then the bar queryset is evaluated and returns an empty queryset. Subsequently when you process the filtering during an actual request the filtered queryset never gets attached to the form field.

[carltongibson]

@shangxiao Any chance you could open a branch with a failing test case so I can see exactly what you mean? Thanks.

[shangxiao]

@carltongibson cheers for responding so quickly 👍 Added a PR with test showing what I meant.

[shangxiao]

Honestly I'm not really expecting a fix per se but just wanted to "document" it with an issue so that others may be aware. Perhaps mentioning this in the docs might be worthwhile?

[carltongibson]

Thanks @shangxiao. Let me have a think about it, but yes... forcing evaluation of base_filters this way may not be something we're going to want to work around. 🤔

[shangxiao]

One thing we might do here is just deepcopy base_filters to introspect it.

[shangxiao]

Actually also worth mentioning that introspecting filters like so works fine:

FooFilterSet().filters['bar'].field