Is there a way to count the number of unique occurrences of a key using the aggregation framework?

[Adam Fields]

If I have a bunch of documents where each document has a category id, I can get the documents per category with:

db.documents.aggregate([{$project:{category_id:1, _id:0}}, {$group:{_id:"$category_id", posts:{ $sum : 1 }}}]);

Is there any way to get the number of unique categories without getting the whole return set and counting the number of documents (and is that even possible with the mongo shell)?

[Stephen Lee]

Adam, have you tried using the distinct command, http://docs.mongodb.org/manual/reference/method/db.collection.distinct/#db.collection.distinct?  Something like db.documents.distinct('category_id').toArray().length?

Both your suggestion, using the aggregation framework, and mine require using the full collection, but using the distinct command should be a more direct query.

[Adam Fields]

On Jan 25, 2013, at 3:23 PM, Stephen Lee <steph...@10gen.com> wrote:

> Adam, have you tried using the distinct command, http://docs.mongodb.org/manual/reference/method/db.collection.distinct/#db.collection.distinct? Something like db.documents.distinct('category_id').toArray().length?
>
> Both your suggestion, using the aggregation framework, and mine require using the full collection, but using the distinct command should be a more direct query.

That looks like it might do exactly what I need. I'll check it out. Thanks!

[Ronald Stalder]

don't use toArray() - just distinct(...).length (distinct returns an array)