id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
14877	ModelFormSet.save() with a deleted form should work even if the model has already been deleted	andornaut	Anssi Kääriäinen	"== Background ==

This issue occurs when you have a `ModelFormSet` that contains a deleted Form that has an already deleted instance. When trying to save the `ModelFormSet`, Django will try and fail during `save()` because it is not able to lookup the instance by the raw_pk_value.

== Use Case ==

An edit page with two formsets. The formset at the top allows you to edit or delete instances of model `Foo`. The formset at the bottom allows you to edit or delete instances of model `Bar`, each of which have a FK to one of the instance of `Foo` in the top formset.

If the user deletes `Foo` above, the the related Bars below are marked as deleted (`fields['DELETE']=True`). If a `Foo` is deleted and save() is called on the top formset before the bottom formset, then the `Bars` that have a FK to the deleted `Foo` will be deleted. Then when `ModelFormSet` tries to delete the `Bar` forms marked as deleted it will fail when it tries to lookup the deleted `Bar` by its PK.

This may can also occur outside of a single request if a Foo is deleted, for example, in another browser tab (untested):

1) Bar edit page GET request.
2) Foo edit page GET request.
3) Foo edit page POST: Delete 1 Foo, which cascade deletes some Bars
4) Bar edit page POST: Delete 1 Bar (the same one deleted in !#3) - fails on `save()` during instance lookup

== Relevant Code ==

django.forms.models.!BaseModelFormSet.save_existing_objects()!#603
{{{
#!python
    def save_existing_objects(self, commit=True):
# ... snip ...
            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)
}}}
== Suggestion ==

It may make more sense to use `form.instance` instead of looking up the instance by its `raw_pk_value` again. This would also require fewer calls to the database.

Perhaps, consider replacing the code above with:
{{{
#!python
obj = self.instance
}}}

Of course, other changes may also be necessary to fix this use case."	Bug	closed	Forms	1.3-alpha	Normal	fixed		patrys@… olau@…	Ready for checkin	1	0	0	0	0	0