'Given a ModelForm return an unsaved instance. ``change`` is True if
the object is being changed, and False if it\'s being added.'

def save_form(self, request, form, change):

    r = form.save(commit=False)
    parent_id = request.GET.get(u'parent_id', None)
    if (not parent_id):
        parent_id = request.POST.get(u'parent_id', None)
    if parent_id:
        parent = Folder.objects.get(id=parent_id)
        r.parent = parent
    return r
