Setting a custom queryset for a ModelChoiceField in init()

Given the following form:

class ListReqStep1(forms.Form):
    group = forms.ModelChoiceField(
        queryset=IETFWG.objects.all().filter(status=IETFWG.ACTIVE).
            select_related(depth=1).order_by('acronym.acronym'),
        required=False, empty_label="-- Select Working Group")
    domain_name = forms.ChoiceField(choices=DOMAIN_CHOICES,
        required=False, widget = forms.Select, initial='ietf.org')
    list_to_close = forms.ModelChoiceField(
        queryset=ImportedMailingList.objects.all(),
        required=False, empty_label="-- Select List To Close")

The list_to_close choices should really depend on the value of the domain_name field (i.e., the lists that are hosted at that domain). The classic way to handle this is to set the choices in __init__(). However, with a ModelChoiceField, there's a twist. The field's choices are a QuerySetIterator that iterates over the queryset when needed (deferring the query until it's definitely needed). Unfortunately, if you just change the form element's queryset in __init__(), that changes what the form field's choices will return, but not what the widget will use. The obvious code:

    def __init__(self, *args, **kwargs):
        super(ListReqStep1, self).__init__(*args, **kwargs)
        # Base the queryset for list_to_close on the initial value
        # for the domain_name field.
        self.fields['list_to_close'].queryset = 
               ImportedMailingList.choices(self.initial.get('domain_name','ietf.org'))

simply results in the original queryset being used, since even though you've changed the queryset, the widget has a copy of the original QuerySetIterator. To get the right choices, you have to give them to the widget again - this seems very magic, but works:

    def __init__(self, *args, **kwargs):
        super(ListReqStep1, self).__init__(*args, **kwargs)
        # Base the queryset for list_to_close on the initial value
        # for the domain_name field.
        self.fields['list_to_close'].queryset = 
               ImportedMailingList.choices(self.initial.get('domain_name','ietf.org'))
        # This is necessary after changing a ModelChoiceField's
        # queryset.
        self.fields['list_to_close'].widget.choices =
             self.fields['list_to_close'].choices