Clarify ModelFormSet's handling of initial data

[Johannes Maron]

If you create a ModelFormSet and pass initial data for some forms, the forms are evaluated as has_changed False.
All in all that is true, but it has the the side effect that the forms in the formset don't get saved, tho they contain data.

The error is here:
​https://github.com/django/django/blob/master/django/forms/models.py#L777-L778

It should not only be checked for has_changed but also for initial data.

[Tim Graham]

You should use initial forms instead of extra forms if you require those populated forms to be submitted. Changing the behavior to validate and save unchanged extra forms would be backwards incompatible. Feel free to reopen if I missed something.

[Johannes Maron]

That does not work for ModelFormSets as the pop initial to initial_extra and only except existing objects using the Queryset.
There currently is no way to create objects using a ModelFormSet with initial data.

I get your point, about compatibility, but you should decide if this odd behavior is a bug or a feature. Personally it feels like a bug. Especially as Model and regular form sets handle initial data so differently. There is no consistent behavior nor any documentation about this.

[Tim Graham]

I had no trouble with this view:

from django.forms.models import modelformset_factory

def test_view(request):
    qs = Poll.objects.none()
    PollFormSet = modelformset_factory(Poll, fields='__all__')
    if request.method == 'POST':
        formset = PollFormSet(request.POST, queryset=qs)
        if formset.is_valid():
            formset.save()
    else:
        formset = PollFormSet(queryset=qs, initial=[
            {'question': 'initial', 'pub_date': timezone.now()}])
    return render(request, 'polls/manage.html', {'formset': formset})

Can you provide a snippet that demonstrates your problem?

[Johannes Maron]

As I said:
It does not work if your submitted are not yet created and you can't pass a queryset.

from django.forms.models import modelformset_factory

def test_view(request):
    qs = Poll.objects.none()
    PollFormSet = modelformset_factory(Poll, fields='__all__')
    if request.method == 'POST':
        formset = PollFormSet(request.POST, inital=[{'vote': 2}, {'vote':4}])
        if formset.is_valid():
            obj_list = formset.save()
            assert len(obj_list) == 2
    else:
        formset = PollFormSet(queryset=qs, initial=[
            {'question': 'initial', 'pub_date': timezone.now()}])
    return render(request, 'polls/manage.html', {'formset': formset})

This will fail.

[Tim Graham]

I'm using the models from the tutorial. Poll doesn't have a vote field also inital is a typo in your example. Maybe you could explain your actual use case instead of modifying my example as I don't understand what the expected behavior or usefulness of that snippet is.

[Johannes Maron]

Sorry, I think I fixed the example.

Let me try to be a little more specific to what's wrong:

There are two ways to pre fill a ModelFormSet, by passing a queryset or initial data. The later is described here:
​https://docs.djangoproject.com/en/dev/topics/forms/formsets/#using-initial-data-with-a-formset

Sadly a ModelFormSet only takes queryset in account for initial_form_count
​https://github.com/django/django/blob/master/django/forms/models.py#L579

Therefore initial data is used for form construction but not accounted for form saving, which is very much unexpected.
​https://github.com/django/django/blob/master/django/forms/models.py#L606-L611

[Johannes Maron]

The fix would be to change
​https://github.com/django/django/blob/master/django/forms/models.py#L777
to:

if not (form.has_changed() or form.initial):

[Tim Graham]

What are you trying to accomplish by passing initial to PollFormSet() along with request.POST? Do you want to override request.POST with the initial values or use them if no value is provided in request.POST?

[Johannes Maron]

Well it should work in a FormView right? The form view passes the initial as well as request.POST.
​https://github.com/django/django/blob/master/django/views/generic/edit.py#L76-L90

[Tim Graham]

If you want the initial data to be considered "changed", then don't pass it to the bound FormSet as in my example. Is there a problem with that solution?

[Johannes Maron]

I know that I can make it work, I'm just asking myself the questions if it shouldn't just work out of the box.

A ModelFormSet would be the only model, that has a different behavior regarding initial data and therefore also doesn't work with the generic FormView.

This really doesn't sound like a feature, it's a bug.

[Tim Graham]

Accepting as a documentation issue. As I said before, I don't think changing the behavior is acceptable given backwards compatibility. For example, your patch assumes that the initial data includes all fields of the model. Since unchanged formsets aren't validated, saving would fail with a database IntegrityError if a required field is omitted.

Let me know if anything is unclear or if you think something additional should be added to the documentation. I've attached a proposed patch.

[Johannes Maron]

Fair enough, thanks.

[Tim Graham <timograham@…>]

In 6de3a1e2:

Fixed #24541 -- Clarified ModelFormSet's handling of initial data.

[Tim Graham <timograham@…>]

In 7c7d29a:

[1.7.x] Fixed #24541 -- Clarified ModelFormSet's handling of initial data.

Backport of 6de3a1e2c34ae5bfcdec3ebbf3d682aa578ecae0 from master

[Tim Graham <timograham@…>]

In a70dea2c:

[1.8.x] Fixed #24541 -- Clarified ModelFormSet's handling of initial data.

Backport of 6de3a1e2c34ae5bfcdec3ebbf3d682aa578ecae0 from master