The db.collection.mapReduce() method delivers similar results to that of the aggregation pipeline. The main difference is that rather than performing operations in stages, map-reduce uses JavaScript functions to produce results. This gives you access to the full programming power that's available in JavaScript. Because it operates outside of the aggregation framework, however, performance is generally worse. If there is a high degree of complexity in your query, it might be worth considering using this feature. Otherwise, the MongoDB documentation recommends using the aggregation pipeline framework.
To demonstrate map-reduce functionality, we will use the same purchases collection that we described previously. Here is the ...
