A QuerySet by aggregate field value

Let's say I have the following model:

class Contest:    title = models.CharField( max_length = 200 )    description = models.TextField()class Image:    title = models.CharField( max_length = 200 )    description = models.TextField()    contest = models.ForeignKey( Contest )    user = models.ForeignKey( User )    def score( self ):        return self.vote_set.all().aggregate( models.Sum( 'value' ) )[ 'value__sum' ]class Vote:    value = models.SmallIntegerField()    user = models.ForeignKey( User )    image = models.ForeignKey( Image )

The users of a site can contribute their images to several contests. Then other users can vote them up or down.

Everything works fine, but now I want to display a page on which users can see all contributions to a certain contest. The images shall be ordered by their score.Therefore I have tried the following:

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

As I feared it doesn't work since 'score' is no database field that could be used in queries.