Schema generation with django-filter ModelMultipleChoiceFilter and to_field_name in 0.29.0

[glennmatthews]

Describe the bug

When using a django-filter ModelMultipleChoiceFilter with a non-primary-key to_field_name, schema generation in 0.29.0 logs a warning that was not seen in previous versions:

    # works, no warning in schema generation
    groups_id = ModelMultipleChoiceFilter(
        field_name="groups",
        queryset=Group.objects.all(),
        label="Group (ID)",
    )
    # works, but logs a warning in schema generation under 0.29.0 - see below
    groups = ModelMultipleChoiceFilter(
        field_name="groups__name",
        queryset=Group.objects.all(),
        to_field_name="name",
        label="Group (name)",
    )

Unable to guess choice types from values, filter method's type hint or find "groups" in model. Defaulting to string.

This appears to be a regression introduced by #1446. Changing the filter definition as follows makes drf-spectacular happy but breaks the operation of the filter:

    groups = ModelMultipleChoiceFilter(
        field_name="groups",  # <------<< removed "__name"
        queryset=Group.objects.all(),
        to_field_name="name",
        label="Group (name)",
    )

ValueError: Field 'id' expected a number but got 'Group 1'.

I believe this is related to an inconsistency in django-filter about how ModelMultipleChoiceFilter behaves when to_field_name is the PK field of the related model, versus when it is not the PK.

See also: carltongibson/django-filter#1661 (comment)

To Reproduce

Expected behavior

Schema generation needs to work correctly and log no warnings with correctly constructed django-filter ModelMultipleChoiceFilter filters, both when to_field_name is the PK field of the related model and when it is not the PK field.