Fix compatibility with django-polymorphic 4.6 (complete)

[djmaze]

As the author of #279 is not responsive, here is a new PR which should fix the remaining problems.

[nattyg93]

@fdintino Any chance we can get this merged and released? With Django 4.2 reaching EOL, this is a blocker for upgrading to Django 5.2 when using django-polymorphic and django-nested-admin.

[fdintino]

The reason I haven't merged this yet is that the logic here is very different than what was in _get_inheritance_relation_fields_and_models.

https://github.com/jazzband/django-polymorphic/blob/d2f9ca27c0bd0ca522e80266f5205a760d7f74d2/src/polymorphic/models.py#L342-L390

I would be more inclined to merge this if the logic was functionally identical. At the very least it needs to be using __subclasses__ since this variable is meant to include child models only. The model parents are already accounted for in a different variable. I think this might be functionally equivalent:

model_cls = self.formset.model.__class__
sub_models = [
    sub_cls
    for sub_cls in model_cls.__subclasses__()
    if model_cls in sub_cls._meta.parents
]

[djmaze]

The reason I haven't merged this yet is that the logic here is very different than what was in _get_inheritance_relation_fields_and_models.

@fdintino Okay, sorry, I was not knowledgeable in this codebase. I just tried to implement your suggested approach, maybe it is more correct now?

[nattyg93]

@fdintino not to harass you, but think you could review this one again, please?

[djmaze]

Interestingly, with Django 6, this seems not a problem anymore.

[theskumar]

@djmaze It's not actually Django 6 that determines this — the trigger is the django-polymorphic version, not the Django version.

_get_inheritance_relation_fields_and_models was removed in django-polymorphic 4.11.0 and is still absent on the current default branch (latest release v4.11.5). The two call sites in nested_admin/polymorphic.py (get_polymorphic_related_models and inline_formset_data) still invoke it, so any polymorphic model with MTI child inlines raises:

AttributeError: '<Model>' object has no attribute '_get_inheritance_relation_fields_and_models'

I just reproduced this on Django 6.0.6 + django-polymorphic 4.11.5 + django-nested-admin 4.1.6 — opening the admin add page for a PolymorphicModel with NestedStackedPolymorphicInline children returns a 500 via inline_formset_data → get_compatible_parents → get_polymorphic_related_models. If a Django-6 setup doesn't hit it, it's almost certainly resolving an older polymorphic (≤4.1, where the method still exists) or has no MTI child models to enumerate.

So this PR is still needed. fdintino's suggested __subclasses__-based logic matches the original method's child-model semantics and is correct. As a sanity check, restoring just that data path (subclasses filtered by model_cls in sub_cls._meta.parents) resolves the 500 and correctly enumerates all child types in our app.