A real-world MongoDB map/reduce example used by the private group mailing lists tool Fiesta explained in detail. The only part I don’t agree with is emphasized below:
Map/Reduce is a great way to do aggregations and ETL-type operations with MongoDB.
Probably nitpicking, but MongoDB’s MapReduce—actually this applies to most NoSQL databases MapReduce implementations: CouchDB, Riak, etc.—can do only the transform part and very less so load and no extract.
Original title and link: A MongoDB Map/Reduce Job Explained ( ©myNoSQL)