Example of a MongoDB map-reduce with a composite key, sorting the result set in the reduce operation. JavaScript example and its correspondent equivalent in Java

command.js

db.books.mapReduce(mapFn, reduceFn, { out: { reduce: 'books_rating'}});

generalDataStructure.txt

{
  _id: {
    rating: 4,
    author: X
  },
  value: [
    {
      title: 'Le petit prince',
      author: 'Antoine de Saint-Exupéry',
      ISBN: 9780440842149,
      description: 'Description'
    },
    ...
  ]
}

map.js

var mapFn = function(){
  var value = {
    title: this.title,
    author: this.author,
    ISBN: this.ISBN,
    description: this.description
  };
  emit({rating: this.rating, author: this.author.id}, value);
};

MapReduce.java

Mongo mongo = new Mongo(hosts);

String mapFn = "function(){ var value = {"+
    "title: this.title,"+
    "author: this.author,"+
    "ISBN: this.ISBN,"+
    "description: this.description"+
  "};"+
  "emit({rating: this.rating, author: this.author.id}, value);"+
"}";

String reduceFn = "function(key, values){"+
  "return values.sort(function(a,b){"+
    "return a.title - b.title;"+
  "});"+
"};";

com.mongodb.MapReduceCommand cmd = new com.mongodb.MapReduceCommand(mongo.getDB("bookdb").getCollection("books"), 
  mapFn, reduceFn, "books_rating", com.mongodb.MapReduceCommand.OutputType.REDUCE, null);


mongo.getDB("bookdb").getCollection("books").mapReduce(cmd);

mapReduce.js

var mapFn = function(){
  var value = {
    title: this.title,
    author: this.author,
    ISBN: this.ISBN,
    description: this.description
  };
  emit({rating: this.rating, author: this.author.id}, value);
};

var reduceFn = function(key, values){
  return values.sort(function(a,b){
    return a.title - b.title;
  });
};

db.books.mapReduce(mapFn, reduceFn, { out: { reduce: 'books_rating'}});

reduce.js

var reduceFn = function(key, values){
  return values.sort(function(a,b){
    return a.title - b.title;
  });
};