Using aggregation in Django with

Question

Following is my model:

class Story(models.Model):
    author = models.ForeignKey(User, related_name='author_id', default=1)
    source = models.ForeignKey(User, related_name='source_id', default=2)

User is the django provided model.

Now I need to find out number of articles authored and sourced by each user.

I thought of using story_set with user object:

res = User.objects.annotate(Count('story_set'))

However, there are two columns in story referencing User. Hence, Django won't know which one to use?

Can anyone help me on this?

Chris Pratt · Accepted Answer · 2011-12-07 17:11:30Z

1

story_set doesn't exist one way or another. That would've been the default related_name if you hadn't provided one. Since you did, you have to use those.

res = User.objects.annotate(Count('author_id'))

OR

res = User.objects.annotate(Count('source_id'))

So, that's how Django knows the difference.

FYI: if you had used the default (so you accessed stories via .story_set.all(), you don't use the "_set" part in queries. It would just be Count('story').

answered Dec 7, 2011 at 17:11

Chris Pratt

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2 Comments

Husain Basrawala Over a year ago

My doubt is how will Django know about author_id attribute if it doesn't exist in User. It exists in Story.

Chris Pratt Over a year ago

That's what related_name does. It defines the relationship on the opposite site. If Story has users then User has story_set. Or, in your case, author_id and source_id (which is why you should change those to something more meaningful like author_set and source_set). This is the magic of Django.

second · Accepted Answer · 2011-12-07 17:12:39Z

1

the reason django makes you specify a related_name is exactly for this reason. you want Count('author_id') instead of Count('story_set') (and you should probably give these better names, e.g. author_set)

answered Dec 7, 2011 at 17:12

second

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U-DON · Accepted Answer · 2011-12-07 17:16:44Z

0

res = User.objects.annotate(num_authored=Count('author_id')).annotate(num_sourced=Count('source_id'))
res[0].num_authored
res[0].num_sourced

If you want both the number of authored and number of sourced articles in one query. You might want more appropriate related names, like "authored" and "sourced".

answered Dec 7, 2011 at 17:16

U-DON

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