python - Django query with an aggregate function -

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i have following models:

class topiclabel(models.model):     name = models.charfield(max_length=256)     order = models.integerfield(null=true, blank=true)     def __unicode__(self):         return self.name      def hastopics():         return topiclabelconnection.objects.filter(labelid=self.id).count() > 0  class topiclabelconnection(models.model):     topicid = models.foreignkey(topic, related_name='connection_topic')     labelid = models.foreignkey(topiclabel, related_name='connection_label')      def __unicode__(self):         return self.labelid.name + ' / ' + self.topicid.title

in view want create list of topiclabels, have @ least 1 connection (i. e. hastopics returns true).

afaik impossible in django use instance methods in filter expressions (i. e. topiclabel.objects.filter(topiclabel.hastopics).order_by('order') impossible).

what correct (django-style) way implement such query (preferably database-independent) ?

for specific case, don't need aggregation function @ all. use isnull filter:

topiclabel.objects.filter(connection_label__isnull=false)

for cases need aggregate, can filter on annotations described in aggregation documentation.


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