python - Validate objects in many-to-many relationship in Django -

i have 2 models group , user. have manytomanyfield in group referencing user. have intermediate model groupuser storing additional data (date assigning , type of membership) many-to-many relationship.

i have page /groups/group-<group_id>/create-user/ @ add users group. problem if user created type 1, cannot assigned other groups. how can validate user not assigned other groups if relationship exists specific user , type 1?

i'm using createview

class groupusercreateview(createview):     model = groupuser     fields = ['user', 'type']     template_name = "group_user_create_form.html"      def dispatch(self, request, *args, **kwargs):         self.group = get_object_or_404(group, id=self.kwargs['group_id'])         return super(groupusercreateview, self).dispatch(request, *args, **kwargs)      def form_valid(self, form):         form.instance.group = self.group         return super(groupusercreateview, self).form_valid(form)      def get_success_url(self):         return reverse('group_user_list', kwargs={'group_id': self.group.id}) 

don't know createview, guess, overwrite get_form , adjust queryset on on user field.

def get_form(self, form_class):     create_form = super(groupusercreateview, self).get_form(form_class)     create_form.fields['user'].queryset = user.objects.exclude(groupuser__type=1)     return create_form