Allowing further overriding of save_formset on a ModelAdmin

Question

Pretty basic usage scenario here. I want to save the user who created an object and the user who last modified it. However, it's an inlined model so I, of course need to use `save_formset`. The Django docs have the following example code: ``` class ArticleAdmin(admin.ModelAdmin): def save_formset(self, request, form, formset, change): instances = formset.save(commit=False) for instance in instances: instance.user = request.user instance.save() formset.save_m2m() ``` The thing is, if you notice, since `super` is never called, this is a dead-end. If the `ModelAdmin` is subclassed and this method is overridden in the same way, you lose the functionality inherent in the parent. This matters because this is such a common usage scenario that I want to factor out the functionality, so I created the following: ``` class TrackableInlineAdminMixin(admin.ModelAdmin): def save_formset(self, request, form, formset, change): instances = formset.save(commit=False) for instance in instances: if hasat…

Accepted Answer

This is a doozie.

I had some fun poking around and it appears all of the action happens here in `django.forms.models.BaseModelFormSet`.

The problem is that `ModelFormSet.save()` deletes instances regardless of of the `commit` flag and does not modify the forms to reflect the deleted state.

If you call `save()` again it iterates over the forms and in `ModelChoiceField` cleaning tries to pull up the referenced ID and throws an invalid choice error.

```
def save_existing_objects(self, commit=True):
    self.changed_objects = []
    self.deleted_objects = []
    if not self.initial_forms:
        return []

saved_instances = []
    for form in self.initial_forms:
        pk_name = self._pk_field.name
        raw_pk_value = form._raw_value(pk_name)

# clean() for different types of PK fields can sometimes return
        # the model instance, and sometimes the PK. Handle either.
        pk_value = form.fields[pk_name].clean(raw_pk_value) 
        pk_value = getattr(pk_value, 'pk', pk_value)

obj = self._existing_object(pk_value)
        if self.can_delete and self._should_delete_form(form):
            self.deleted_objects.append(obj)
            obj.delete()  
            # problem here causes `clean` 6 lines up to fail next round

# patched line here for future save()
            # to not attempt a second delete
            self.forms.remove(form)
```

The only way I was able to fix this is to patch `BaseModelFormset.save_existing_objects` to remove the form from `self.forms` if an object is deleted.

Did some testing and there doesn't appear to be any ill effects.

Allowing further overriding of save_formset on a ModelAdmin

Solution

Problem