sort django queryset by latest instance of a subset of related model

Question

I have an Order model and order_event model. Each order_event has a foreignkey to order. so from an order instance i can get: myorder.order_event_set. I want to

Answer 1

Greg's answer is great, and I used it for awhile. However, I wanted to access other fields of latest_event. So, I ended up storing the latest_event as a field of Order:

# The latest event
latest_event = db.ForeignKey('whateverapp.Event', editable=False, null=True)

Then I set up a signal to recalculate the latest event upon save:

@receiver(post_save, sender=Event)
def visit_calculate_latest(sender, instance, **kwargs):
    """Recalculate latest event"""
    instance.parent.latest_event = instance.parent.get_latest_event()

where get_latest_event() did the query:

def get_latest_event(self):
    """Return most recent event or None."""
    try:
        return Event.objects.filter(
            parent=self.id).order_by('-event_date', '-id')[0]
    except IndexError:
        return None

Now I can filter by latest_event__whatever:

    return qs.filter(qobjs).order_by('-latest_event__event_date',
        '-latest_event__pk')