Docs are incorrect about filtering by a Model[Multiple]ChoiceFilter

[george-silva]

In the current documentation we have this following example:

class FooFilter(BaseFilterSet):
    foo = django_filters.filters.ModelMultipleChoiceFilter(
        field_name='attr__uuid',
        to_field_name='uuid',
        queryset=Foo.objects.all(),
    )

However, it's not necessary to set the filter field_name to point to the uuid field. The only needed step is to add the to_field_name and point to the correct relationship.

In my case I've tried doing what is described above, but this fails here:

location: django_filters.filters.ChoiceFilter#158

class ChoiceFilter(Filter):
    field_class = ChoiceField

    def __init__(self, *args, **kwargs):
        self.null_value = kwargs.get('null_value', settings.NULL_CHOICE_VALUE)
        super().__init__(*args, **kwargs)

    def filter(self, qs, value):
        if value != self.null_value:
            return super().filter(qs, value)

        qs = self.get_method(qs)(**{'%s__%s' % (self.field_name, self.lookup_expr): None})
        return qs.distinct() if self.distinct else qs

The reason it fails is that value in this particular code location is already the related model. If you use related_model__uuid as the field name, the lookup becomes: related_model__uuid and value is the model itself. Django fails when it tries to convert the related model instance to an UUID.

The only necessary change seems to be changing the field_name to remove the extra __uuid.

[nils-van-zuijlen]

Thank you for your issue, I just bumped in the same issue, this also solved it for me.

[george-silva]

Created a simple pull request fixing the example.

[medihack]

That is strange. I stumbled across this issue by chance, but I have to add the suffix to the field_name:

This works in my case:

modalities = django_filters.ModelMultipleChoiceFilter(
    queryset=Modality.objects.order_by("code"),
    field_name="modalities__code",
    to_field_name="code",
)

But this doesn't:

modalities = django_filters.ModelMultipleChoiceFilter(
    queryset=Modality.objects.order_by("code"),
    field_name="modalities",
    to_field_name="code",
)

(Error: Field 'id' expected a number but got 'CT'., where CT is a unique code).

[gabn88]

I also struggle with this.

IMO all "multiple" filters should allow all three syntaxes defined in #548. Now the __in requires comma-seperated values, whereas the ModelMultipleChoiceFilter requires the a=b&a=c syntax.

I tried to partly fix this with #1686, but that one seems to have gone stale.

[glennmatthews]

If I'm understanding the code right, if to_field_name is a field other than the PK field of the target model, it is necessary to specify the field name in field_name as well:

modalities = django_filters.ModelMultipleChoiceFilter(
    queryset=Modality.objects.order_by("code"),
    field_name="modalities__code",     # NOT "modalities", that won't work
    to_field_name="code",
)

whereas if the to_field_name is the PK field, this is unnecessary because Django understands that queryset.filter(value=<PK>) is equivalent to queryset.filter(value__pk=<PK>):

    class FooFilter(BaseFilterSet):
        foo = django_filters.filters.ModelMultipleChoiceFilter(
            field_name='attr',  # NOT "attr__uuid", that won't work
            to_field_name='uuid',  # could also omit this entirely I think?
            queryset=Foo.objects.all(),
        )

This seems somewhat inconsistent and confusing to me - perhaps it'd be possible to fix so that both scenarios support both variants?