gistfile1.txt

> var map = function() { emit(this._id, {  new_message_count : this.new_message_count }); };      
> var reduce = function(key, values) {
...       var new_message_sum = 0;
...       values.forEach(function(doc) {
...         new_message_sum += doc.new_message_count
...       });
...       return { stats: { new_message_count: new_message_sum } };
...     };

> var op = db.mail_reader_stats.mapReduce(map, reduce);

> db[op.result].find();                                
{ "_id" : ObjectId("4c76a3753fe2262a83000001"), "value" : { "stats" : { "new_message_count" : 5 } } }
{ "_id" : ObjectId("4c76e62a3fe2260e03000001"), "value" : { "stats" : { "new_message_count" : 2 } } }