update a field of a related table on form save

I have three tables. The first one is application which has a flag for if an app has been submitted as well as created date and modified date. The other two tables relate to the application table. I have two forms that get update those tables. What I want to do is update the application modified date from the save on the other forms.

I have this in the Application model

modified    = models.DateTimeField(auto_now_add=True)

but since it is the other tables I am saving to, I'm not sure how to update that field whenever the other two tables update.

My view for one of the forms looks like this:

class AnswersUpdate(UpdateView):

    model = Answers

    form_class = AnswersForm

    slug_field = 'created_by'

    template_name = 'requestform/applicant_form.html'

    @method_decorator(login_required(login_url='/accounts/login/'))

    def dispatch(self, *args, **kwargs):

        return super(AnswersUpdate, self).dispatch(*args, **kwargs)

    def get_object(self, queryset=None):

        return Answers.objects.get(created_by=self.request.user)

    def form_valid(self, form):

        obj = form.save(commit=False)

        obj.created_by = self.request.user

        obj.save()

        a = Application.objects.get(created_by=self.request.user)

       ## how do I save to the Application here?

        return HttpResponseRedirect(reverse('requestform:app_check', kwargs={'app_id': obj.id}))

Any help would be appreciated.

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