Neoliz

Discussion.objects.all().annotate(
                comment_old_date=Max(
                    Case(
                        When(
                            comments__date_create__isnull=True,
                            then=datetime.datetime.strptime('1950-01-01 00:00:00', '%Y-%m-%d %H:%M:%S')
                        ),
                        default='comments__date_create'
                    )
                )
            ).order_by('-comment_old_date')