views.py

class ConfirmView(UpdateView):
    template_name = 'rsvp/confirm.html'
    success_url = reverse_lazy('rsvp:thanks')
    form_class = InviteeForm
    model = Invitee

    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)
        obj = self.get_object()
        num_current_extras = Extra.objects.filter(invitee=obj).count()
        ExtraFormSet = modelformset_factory(Extra, ExtraForm, extra=obj.allowed_extras-num_current_extras)

        context['extras_form'] = ExtraFormSet()

        return context

    def get_object(self, queryset=None):
        return Invitee.objects.get(code=self.request.session['code'])

    def post(self, request, *args, **kwargs):
        self.object = self.get_object()
        form = self.get_form()

        ExtraFormSet = modelformset_factory(Extra, ExtraForm, extra=self.object.allowed_extras)
        extra_formset = ExtraFormSet(self.request.POST)

        if form.is_valid() and extra_formset.is_valid():
            extras = extra_formset.save(commit=False)
            for extra in extras:
                extra.invitee = self.object
                extra.save()
            self.object = form.save()
            return HttpResponseRedirect(self.get_success_url())
        elif not extra_formset.is_valid():
            return self.form_invalid(extra_formset)
        else:
            return self.form_invalid(form)