Hi,
is it possible to constrain a Count aggregate by adding some extra
filters?
I have a model which has a self referentiell foreign key.
class Topic(models.Model):
...
topic = models.ForeignKey('self',
related_name='subtopics', null=True, blank=True)
...
Now I would like to select a list of Topic instances which is
annotated with the count of subtopics. This works like a charm with
the following statement.
Topic.objects.all().annotate(Count('subtopics'))
But can I constrain the subtopics which are counted? Topics have an
attribute trashed_at and complete_at. I only want the count the
subtopics, where these attributes are null.
Currently, I do this with extra and a custom SQL query. But the source
of the aggregates suggest I can add a some extra constraints.
Can anybody supply a working example?
Best regards,
Oliver
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