In our previous recipe, Implementing aggregation in Mongo using a Java client, we saw how to execute aggregation operations in Mongo using the Java client. In this recipe, we will work on the same use case as we did for the aggregation operation but We will use MapReduce. The intent is to aggregate the data based on the state names and get the top five state names by the number of documents that they appear in.
If somebody is not aware of how to write MapReduce code for Mongo from a programming language client and is seeing it for the first time, you might be surprised to see how it is actually done. You might have imagined that you would be writing the map and reduce function in the programming language that you are writing the code in, Java in this case, and then using it to execute the map reduce. However, we need to bear in mind that the MapReduce jobs run on the mongo servers and they execute JavaScript functions. Hence, irrespective of...