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
18508	Bug in handling out-of-date POST data with inlineformset_factory	Ole Laursen	Anssi Kääriäinen	"Hi, there's a bug in BaseModelFormSet._construct_form. There's a guard supposed to take care of bound forms, but if one of the entries has been deleted, self._existing_object(pk) will return None so you can end up in

{{{
        if i < self.initial_form_count() and not kwargs.get('instance'):
            kwargs['instance'] = self.get_queryset()[i]
}}}

which is dangerous for a bound form where you must use ids to identify elements since element ""i"" can have changed from when the form was generated. In my case, I got an IndexError because there weren't enough elements left from a recent deletion.

I think the correct fix is to change the ""if"" to an ""elif"".

I've attached a patch with this fix and a test case for reproducing the problem (there doesn't seem to be any tests of model formsets?).

Note that the test as entered doesn't pass even with the fix because of issue #14877 - if you delete formset.save() and below in the test, it does pass."	Bug	closed	Forms	1.4	Normal	fixed			Accepted	1	0	0	0	0	0