Question

How do I access the output from the following mongoDB map reduce code? I assume that the map reduce function is producing a collection called ‘session_stat’ with fields: ‘dayOfWeek’ and ‘count’ that I can print using find() and my printValue function. But when I execute the code using ‘mongo code.js’, I just get “undefined”. Thanks in advance.

db = db.getSiblingDB("mrex");

// simulate saving records that log the lengths of user sessions in seconds                                                                                                                                                                                                                                                                                                 
db.session.save({userid:"a", time: ISODate('2011-11-03 14:17:00'), length: 95});
db.session.save({userid:"b", time: ISODate('2011-11-03 14:23:00'), length: 110});
db.session.save({userid:"c", time: ISODate('2011-11-03 15:02:00'), length: 120});
db.session.save({userid:"d", time: ISODate('2011-11-03 16:45:00'), length: 45});

function mapf()
{
    emit(this.time.getDay(),
     {count:1});
}

function reducef(key, values)
{
    var r = {dayOfWeek:key, count:0};
    values.forEach(function(v)
               {
                   r.count += v.count;
               });
    return r;
}

var mrcom1 = db.runCommand( { mapreduce:"session",
                          map:mapf,
                          reduce:reducef,
                          query: {time: {$gt:ISODate('2011-11-03 00:00:00')}},
                          out: { reduce: "session_stat" }
});

function printValue (r) {
    print(r.count)
};

db.session_stat.find().forEach(printValue)

Answer 1

MongoDB’s map-reduce command generates a set of documents with a field named “value”, which contains the reduce return value. You’ll need to reach inside this field to find your subdocument.

Use this instead as your printValue function:

function printValue (r) {
    print(r.value.count)
};