Working with Map-Reduce functions

Prior to the introduction of the aggregation framework, in order to perform grouping and other aggregation operations, database users used a less efficient method: db.collection.mapReduce(). This command has been a part of the MongoDB command set since the beginning, but is more difficult to implement and maintain and suffers from relatively poor performance compared with similar operations using the aggregation framework. Let's now have a look at the mapReduce() method.