Polymorphic child model discovery fails for deeply nested or abstract class hierarchies

[MBI-joshquercia]

Problem

The current implementation of NestedPolymorphicInlineAdminFormset.inline_formset_data() uses self.formset.model()._get_inheritance_relation_fields_and_models() to discover child polymorphic models. This method has limitations:

1. It doesn't discover child models at arbitrary inheritance depths
2. It doesn't correctly handle abstract polymorphic base classes
3. It returns deferred model classes in some querysets (when using .only())

Proposed Solution

Replace the child model discovery with two new helper functions:

• get_all_subclasses(python_class) - Recursively finds all subclasses at any depth
• get_child_concrete_polymorphic_models(base_model) - Filters to only concrete (non-abstract) polymorphic models, excluding deferred classes

Implementation

I have a working implementation with tests in my fork:
https://github.com/jintb/django-nested-admin

Key changes in nested_admin/polymorphic.py:

• Added get_all_subclasses() helper function
• Added get_child_concrete_polymorphic_models() function
• Updated the call site in NestedPolymorphicInlineAdminFormset.inline_formset_data()

Tests added in nested_admin/tests/nested_polymorphic/test_polymorphic_abstract_classes/

Happy to open a PR if there's interest in this fix.

[MBI-joshquercia]

Also handled by this PR which I forked off of #228

[fdintino]

Deferred model classes haven't existed since Django 1.10, so I don't think that's a concern (the snippet upon which your changes were based is from 11 years ago, so it makes sense that it would have handling for that). And I don't think there is actually an issue having to do with deep nesting; the data attribute added to an inline only affects the behavior of its children. And those children must necessarily come from related descriptors in Django, which is the thing that _get_inheritance_relation_fields_and_models uses. However, the filtering for abstract models makes sense to me.

In light of the removal of _get_inheritance_relation_fields_and_models from django-polymorphic as part of a refactor, I'm inclined to inline it here and then add the concrete filtering on top of that. I will test that approach against the tests in your fork.