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A MongoDB Map/Reduce Job Explained

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[1] and no extract.

  1. One could argue that MongoDB’s out option can be seen as equivalent to the load phase, but we can agree that having the results replacing or merged in a collection is just a use case  

Original title and link: A MongoDB Map/Reduce Job Explained (NoSQL database©myNoSQL)