Django Count() in multiple annotations

问题:

Say I have a simple forum model:

class User(models.Model):
    username = models.CharField(max_length=25)
    ...

class Topic(models.Model):
    user = models.ForeignKey(User)
    ...

class Post(models.Model):
    user = models.ForeignKey(User)
    ...

Now say I want to see how many topics and posts each users of subset of users has (e.g. their username starts with "ab").

So if I do one query for each post and topic:

User.objects.filter(username_startswith="ab")
            .annotate(posts=Count('post'))
            .values_list("username","posts")

Yeilds:

[('abe', 5),('abby', 12),...]

and

User.objects.filter(username_startswith="ab")
            .annotate(topics=Count('topic'))
            .values_list("username","topics")

Yields:

[('abe', 2),('abby', 6),...]

HOWEVER, when I try annotating both to get one list, I get something strange:

User.objects.filter(username_startswith="ab")
            .annotate(posts=Count('post'))
            .annotate(topics=Count('topic'))
            .values_list("username","posts", "topics")

Yields:

[('abe', 10, 10),('abby', 72, 72),...]

Why are the topics and posts multiplied together? I expected this:

[('abe', 5, 2),('abby', 12, 6),...]

What would be the best way of getting the correct list?

回答1:

I think Count('topics', distinct=True) should do the right thing. That will use COUNT(DISTINCT topic.id) instead of COUNT(topic.id) to avoid duplicates.

User.objects.filter(
    username_startswith="ab").annotate(
    posts=Count('post', distinct=True)).annotate(
    topics=Count('topic', distinct=True)).values_list(
    "username","posts", "topics")

回答2:

Try adding distinct to your last queryset:

User.objects.filter(
    username_startswith="ab").annotate(
    posts=Count('post')).annotate(
    topics=Count('topic')).values_list(
    "username","posts", "topics").distinct()

See https://docs.djangoproject.com/en/1.3/ref/models/querysets/#distinct for more details, but basically you're getting duplicate rows because the annotations span multiple tables.