DocumentFormSet modification leads to NotUniqueError

[MisterFreeze]

First, let me thank you for this nice library.

I am migrating a project from django models to django mongoengine documents and I am using django-mongodbforms 0.3 as a replacement for the django forms for models. The migration is seamless so far.

I just tried to update a set of forms constructed from a supplied queryset in this way:

    formset_constructor = document_formset_factory(document_class, max_num=1, **formset_kwargs)
    qs = docucment_class.objects.filter(id=obj_id)
    formset_check = formset_constructor(request.POST, queryset=qs)
    if formset_check.is_valid():
        for inst in formset_check.save(commit=False):
            # do something
            inst.save()

The document class has a unique constraint on a field and the inst.save() raises a NotUniqueError (for that field, if value is unchanged).

I think this happens because the instance is freshly constructed during the is_valid() execution and the id value is not copied to that new instance in construct_instance(), which is called by _post_clean().
In fields_for_document(), any ObjectIdField is skipped. ObjectIdFields are skipped in construct_instance() as well. When I remove

    if isinstance(f, ObjectIdField):
        continue

in both functions, the inst.save() succeeds without raising that NotUniqueError.

I think that this is a common use case. Did I do something wrong here?

Best regards,

Chris

[MisterFreeze]

Seriously, did I do something wrong here or have an incorrect understanding of how this should work? If not, I see this as a bug that I'm very wliling to create a pull request for.