Django: How to annotate query with Count of distinct occurrences of field combination?

Question

Ideally I would like to use the Count aggregator to count distinct appearances of two fields, instead of just one:

q = Tag.objects.annotate(success=Count(['artifact__event__session', 'artifact'], distinct=True))

but this does not seem to be allowed. I came up with the hacky solution of concatenating the fields I want to count the distinct combinations of:

q = Tag.objects.annotate(success=Count(Concat('artifact__event__session', 'artifact'), distinct=True))

But this does not seem like the right thing to do. Is there a better way?

---

More background information:

The actual query is more like:

q = Tag.objects.annotate(success=Count(Concat('artifact__event__session', 'artifact'), filter=Q(artifact__event__success=True, artifact__event__user=user), distinct=True))

which has the meaning:

"For each tag, give me the number of distinct session-artifact combinations of artifacts that are tagged with that tag and sessions that included that artifact, where an event that belongs to this user was successful"

with the models being:

class Artifact(models.Model):
    tags = models.ManyToManyField(Tag)
    ...

class Event(models.Model):
    artifact = ForeignKey(Artifact)
    session = ForeignKey(Session)
    success = BooleanField()
    user = ForeignKey(User)
    ...

class Session(models.Model):
    ...

question from:https://stackoverflow.com/questions/65904151/django-how-to-annotate-query-with-count-of-distinct-occurrences-of-field-combin

Answer 1

Waitting for answers