Does Mongoid have Map/Reduce?

I am using Ruby code to calculate sum from the array returned by Mongoid.

But maybe using Map/Reduce can be faster, except I don't see any docs for Map Reduce on mongoid.org and Google for

map reduce site:mongoid.org

doesn't give any result either. (or using MapReduce or Map/Reduce)

There are docs on MongoDB's site

map reduce site:mongodb.org

but need to use Map Reduce with Mongoid as well.