Question

When using map/reduce on MongoDb 2.0.1, it seems that the finalize method is not always called with the same arguments.

I am doing a regular counting + average at the end.

var m = function() {
    emit(this.testName, {
        worked: Number(this.testStatus == "WORKED"),
        failed: Number(this.testStatus == "FAILED"),
        done: Number(this.testStatus == "DONE"),
        count: 1
    });
}

var r = function(key, values) {
    var result = {
        worked: 0,
        failed: 0,
        done: 0,
        count: 0
    }

    values.forEach(function(value) {
      result.worked += value.worked;
      result.failed += value.failed;
      result.done += value.done;
      result.count += value.count;
    });

    return result;
}

var f = function(key, value) {
    data = value || key;
    data.workedMean = data.worked / data.count;
    return data;
}

var cmd = {
    mapreduce: "tests",
    map: m,
    reduce: r,
    finalize: f,
    out: {
        reduce: "fromMongo"
    },
    jsMode: true
}

When the fromMongo collection is empty, f() is called with only one argument, the value. When fromMongo already has values (notice that I use reduce as my out map/reduce parameter), the f() method get two arguments: key and value.

Is that a known behavior?
I managed two make it work using data = value || key;, but I don’t think this is the solution.

Answer 1

this problem is due to a difference in finalize call between the jsMode and non-jsMode (which happens during the post processing of “reduce” mode).
The workaround is to not use jsMode:true, in which case it should always be called with (key, value).

Created a server issue:
https://jira.mongodb.org/browse/SERVER-4535