Sometimes we want to get a distinct count of fields, but not the distinct values themselves. So we do Comment.distinct(:author_id).size. This works, but requires loading the whole array of author_ids into memory first which doesn't scale quite as well.

I was curious if there was a convenient way to do this all in the db.

It seems doable with a custom aggregation or map reduce, where you do the .size in the reduce step. or pipeline. But I was just curious if Mongoid had an easier built in way to do this. Thanks!