MongoDB's MapReduce function in Java returns null

Question

I've some test data in my MongoDB:

> db.test2.find()
{ "_id" : ObjectId("4e76ad8e9d7dc2438ab63dbb"), "name" : "John", "number" : 2 }
{ "_id" : ObjectId("4e76ad8e9d7dc2438ab63dbc"), "name" : "Jane", "number" : 1 }
{ "_id" : ObjectId("4e76ad8e9d7dc2438ab63dbd"), "name" : "John", "number" : 2 }
{ "_id" : ObjectId("4e76ad8e9d7dc2438ab63dbe"), "name" : "Jane", "number" : 1 }

And when I use following map/reduce function, MongoDB return the correct answer.

> m = function() { emit(this.name, {count: 1, sum: this.number}); }

> r = function(key, values) {
    var n = { count: 0, sum: 0};
    for (var i=0; i<values.length; i++) {
        n.sum += values[i].sum;
        n.count += values[i].count;
    }
    return n;
}

> db.test2.mapReduce(m, r, {out: 'test_col2'})
> db.test_col2.find()
{ "_id" : "Jane", "value" : { "count" : 2, "sum" : 2 } }
{ "_id" : "John", "value" : { "count" : 2, "sum" : 4 } }

But If I test a following Java program, the result value is NULL.

public void run() throws Exception {
    m = new Mongo(HOST, PORT);
    db = m.getDB("test");

    DBCollection col = db.getCollection("test2");

    StringBuilder sbMap = new StringBuilder();
    sbMap.append("function() {");
    sbMap.append("    emit(this.name, {count:1, sum:this.number});");
    sbMap.append("}");

    StringBuilder sbReduce = new StringBuilder();
    sbMap.append("function(key, values) {");
    sbMap.append("    var n = { count: 0, sum: 0};");
    sbMap.append("    for (var i=0; i<values.length; i++) {");
    sbMap.append("        n.count += values[i].count;");
    sbMap.append("        n.sum += values[i].sum;");
    sbMap.append("    }");
    sbMap.append("    return n;");
    sbMap.append("}");

    MapReduceCommand cmd = new MapReduceCommand(col, sbMap.toString(), sbReduce.toString(), null,
                    MapReduceCommand.OutputType.INLINE, null);

    MapReduceOutput out = col.mapReduce(cmd);

    for (DBObject o : out.results()) {
        System.out.println( o.toString() );
    }
}

Result:

{ "_id" : "Jane" , "value" :  null }
{ "_id" : "John" , "value" :  null }

What's the problem? I can't understand it. :(

I think the variable names in emit() need to map to variable names in the reduce. — Jonathan Hendler
– Jonathan Hendler, Commented Sep 28, 2011 at 18:32
@JonathanHendler I don't think that is the case. I have the same issue and can copy the db command out of the Java debugger, run it in the mongo console and get the correct results. — matt.franklin
– matt.franklin, Commented Dec 29, 2012 at 3:21

dlaidlaw · Accepted Answer · 2011-10-11 18:05:06Z

2

It is because after you define the sbReduce variable, you never add anything to it. You mistakenly add the function to the sbMap variable. For example, the first line of your reduce function is: sbMap.append(...) where it should be sbReduce.append(...).

answered Oct 11, 2011 at 18:05

dlaidlaw

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