Conditionally changing ModelAdmin inlines based on object's field breaks when changing object and new inlines should appear.

[joeli]

Minimal example:

# models.py
class Parent(models.Model):
    show_inlines = models.BooleanField(default=False)

class Child(models.Model):
    parent = models.ForeignKey(Parent, on_delete=models.CASCADE)


# admin.py
class ChildInline(admin.StackedInline):
    model = Child

@admin.register(Parent)
class ParentAdmin(admin.ModelAdmin):
    def get_inlines(self, request, obj):
        if obj is not None and obj.show_inlines:
            return [ChildInline]
        return []

• Create Parent objects in either initial state and it works as you'd expect, where ChildInline is rendered when show_inlines is True.
• When show_inlines is True, you can also set it to False from the admin site, and the ChildInline disappears as expected.
• But when show_inlines is False, you cannot re-enable it. Saving the object fails due to a validation error in the new ChildInline that didn't exist before saving:

(Hidden field TOTAL_FORMS) This field is required.
(Hidden field INITIAL_FORMS) This field is required.
ManagementForm data is missing or has been tampered with. Missing fields: child_set-TOTAL_FORMS, child_set-INITIAL_FORMS. You may need to file a bug report if the issue persists.

[Mariusz Felisiak]

Thanks for the report. I was able to fix this issue by passing an old instance to the get_inlines() 🤔:

diff --git a/django/contrib/admin/options.py b/django/contrib/admin/options.py
index 1e5fab917e..7b2b893c10 100644
--- a/django/contrib/admin/options.py
+++ b/django/contrib/admin/options.py
@@ -1585,12 +1585,16 @@ class ModelAdmin(BaseModelAdmin):
         )
         if request.method == 'POST':
             form = ModelForm(request.POST, request.FILES, instance=obj)
+            formsets, inline_instances = self._create_formsets(
+                request,
+                form.instance if add else obj,
+                change=not add,
+            )
             form_validated = form.is_valid()
             if form_validated:
                 new_object = self.save_form(request, form, change=not add)
             else:
                 new_object = form.instance
-            formsets, inline_instances = self._create_formsets(request, new_object, change=not add)
             if all_valid(formsets) and form_validated:
                 self.save_model(request, new_object, form, not add)
                 self.save_related(request, form, formsets, not add)

[Hasan Ramezani]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 2f0f30f:

Fixed #33111 -- Fixed passing object to ModelAdmin.get_inlines() when editing in admin change view.

ModelAdmin.get_inlines() should get an unmutated object when creating
formsets during POST request.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 668b990b:

[4.0.x] Fixed #33111 -- Fixed passing object to ModelAdmin.get_inlines() when editing in admin change view.

ModelAdmin.get_inlines() should get an unmutated object when creating
formsets during POST request.

Backport of 2f0f30f973363a59c20f204f9351724fb2ce7327 from main