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POMongoDB's MapReduce function in Java returns null
 

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  1. POMongoDB's MapReduce function in Java returns null
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    <p>I've some test data in my MongoDB:</p> <pre><code>&gt; 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 } </code></pre> <p>And when I use following map/reduce function, MongoDB return the correct answer.</p> <pre><code>&gt; m = function() { emit(this.name, {count: 1, sum: this.number}); } &gt; r = function(key, values) { var n = { count: 0, sum: 0}; for (var i=0; i&lt;values.length; i++) { n.sum += values[i].sum; n.count += values[i].count; } return n; } &gt; db.test2.mapReduce(m, r, {out: 'test_col2'}) &gt; db.test_col2.find() { "_id" : "Jane", "value" : { "count" : 2, "sum" : 2 } } { "_id" : "John", "value" : { "count" : 2, "sum" : 4 } } </code></pre> <p>But If I test a following Java program, the result value is NULL.</p> <pre><code>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&lt;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() ); } } </code></pre> <p>Result:</p> <pre><code>{ "_id" : "Jane" , "value" : null } { "_id" : "John" , "value" : null } </code></pre> <p>What's the problem? I can't understand it. :(</p>
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    1. USJoachim Sauer
    1. USSteve Hwang
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    2. VO
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    1. COI think the variable names in emit() need to map to variable names in the reduce.
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      1. USJonathan Hendler
    2. CO@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.
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      1. USmatt.franklin
 

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