A QuerySet by aggregate field value

Oh, of course I forget about new aggregation support in Django and its annotate functionality.

So query may look like this:

Contest.objects.get(pk=id).image_set.annotate(score=Sum('vote__value')).order_by( 'score' )

The db-level order_by cannot sort queryset by model's python method.

The solution is to introduce score field to Image model and recalculate it on every Vote update. Some sort of denormalization. When you will can to sort by it.

You can write your own sort in Python very simply.

def getScore( anObject ):
    return anObject.score()
objects= list(Contest.objects.get( pk = id ).image_set)
objects.sort( key=getScore )

This works nicely because we sorted the list, which we're going to provide to the template.