ModelAdmin.list_display: AttributeError when a __ lookup path ends in a relation field (regression from #36926)

[RobKuipers]

Since #36926, list_display entries using __ lookup syntax to traverse related fields (documented at ​https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.list_display) raise AttributeError when the *final* segment of the path is itself a relation field (ForeignKey/OneToOneField), rather than a scalar field.

This worked correctly before #36926 and is not limited to any unusual setup — any related__fk_field entry regresses.

Minimal reproduction

class Publisher(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    title = models.CharField(max_length=100)
    publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE)

class Author(models.Model):
    name = models.CharField(max_length=100)
    book = models.ForeignKey(Book, on_delete=models.CASCADE)

@admin.register(Author)
class AuthorAdmin(admin.ModelAdmin):
    list_display = ["name", "book__publisher"]

Visiting the Author changelist with at least one row raises:

AttributeError: 'Author' object has no attribute 'publisher'

Root cause

#36926 added this to lookup_field() in django/contrib/admin/utils.py,
specifically to let display_for_field() recognize BooleanFields reached
via a __ path so it can render the boolean icon:

                    # The final field is needed for displaying boolean icons.
                    if LOOKUP_SEP in name:
                        f = get_fields_from_path(opts.model, name)[-1]

This sets f unconditionally whenever the path contains LOOKUP_SEP, not only when the terminal field is a BooleanField. Previously, f stayed
None for this whole branch (the "method, property, related field, or callable" fallback), and items_for_result() in django/contrib/admin/templatetags/admin_list.py used the already correctly-traversed value via display_for_value().

Now that f is non-None, items_for_result() takes a different branch that assumes f is a field belonging to result directly:

            else:
                if isinstance(f.remote_field, models.ManyToOneRel):
                    field_val = getattr(result, f.name)

That assumption holds for the normal case (list_display = ["publisher"] where publisher really is a field on the model being rendered), but not for the __-traversal case, where f is a field on the *related* model several hops down the path — result (an Author) has no publisher attribute; result.book does.

Suggested fix direction

Scope the new f assignment to its stated purpose — only set it when the terminal field is actually a BooleanField — rather than for every LOOKUP_SEP-containing path:

                    if LOOKUP_SEP in name:
                        final_field = get_fields_from_path(opts.model, name)[-1]
                        if isinstance(final_field, models.BooleanField):
                            f = final_field

This preserves the boolean-icon feature #36926 added while restoring the pre-6.1 (and documented) behavior for __ paths ending in any other field
type, including relations.

I have not attempted a full patch/tests — that's a little out of my league I'm afraid.