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):
        """
        Given a ModelForm return an unsaved instance. ``change`` is True if
        the object is being changed, and False if it's being added.
        """
        r = form.save(commit=False)
        parent_id = request.GET.get('parent_id', None)
        if not parent_id:
            parent_id = request.POST.get('parent_id', None)
        if parent_id:
            parent = self.get_queryset(request).get(id=parent_id)
            r.parent = parent
        return r