How can you filter model objects by attributes of their supermodel?

Question

To get started with Django I did the intro provided by Django itself. In part 5 of it, you write some tests for views and models. After that, they provide you with ideas for more tests. There is where my problem starts. They suggest you to only show Questions with a number of Choices greater than 0. I have no idea how to do that. My current code can be found at https://github.com/byTreneib/django.git. The Django tutorial can be found at https://docs.djangoproject.com/en/3.0/intro/tutorial05/

models.py

from django.db import models
from django.utils import timezone
import datetime


class Question(models.Model):
    question_text = models.CharField(max_length=200)
    pub_date = models.DateTimeField('date published')

    def __str__(self):
        return self.question_text

    def was_published_recently(self):
        return timezone.now() >= self.pub_date >= timezone.now() - datetime.timedelta(days=1)


class Choice(models.Model):
    question = models.ForeignKey(Question, on_delete=models.CASCADE)
    choice_text = models.CharField(max_length=200)
    votes = models.IntegerField(default=0)

    def __str__(self):
        return self.choice_text

views.py

from django.shortcuts import render
from django.http import HttpResponse, Http404, HttpResponseRedirect
from django.template import loader
from django.shortcuts import render, get_object_or_404
from django.db.models import F
from .models import Question, Choice
from django.urls import reverse
from django.views import generic
from django.utils import timezone


class IndexView(generic.ListView):
    template_name = 'polls/index.html'
    context_object_name = 'latest_question_list'

    def get_queryset(self):
        return Question.objects.filter(pub_date__lte=timezone.now()).filter().order_by('-pub_date')[:5]


class DetailView(generic.DetailView):
    model = Question
    template_name = 'polls/detail.html'

    def get_queryset(self):
        return Question.objects.filter(pub_date__lte=timezone.now())


class ResultsView(generic.DetailView):
    model = Question
    template_name = 'polls/results.html'

    def get_queryset(self):
        return Question.objects.filter(pub_date__lte=timezone.now())


def vote(request, question_id):
    question = get_object_or_404(Question, pk=question_id)
    try:
        selected_choice = question.choice_set.get(pk=request.POST['choice'])
    except (KeyError, Choice.DoesNotExist):
        return render(request, 'polls/detail.html', {'question': question, 'error_message': "No choice selected!"})
    else:
        selected_choice.votes = F('votes') + 1
        selected_choice.save()
        return HttpResponseRedirect(reverse('polls:results', args=(question.id, )))

Answer 1

You can see all Choice instances pointing to a Question using django's RelatedManager. Read more here.

The view would be something like this:

class ListView(generic.ListView):
    model = Question

    def get_queryset(self):
        return Question.objects.filter(choice__isnull = False)