Scope work strangely in mapReduce of MongoDB for the purpose of producing cumulative frequency - mongodb

Question

I have a collection called user, and I want to get cumulative frequency of number of users by date based on the _id field. The desired result should be something like that:
{
{_id: 2013-12-02, value: 10}, //upto 2013-12-02 there are 10 users
{_id: 2014-01-05, value: 20}, //upto 2014-01-05 there are totally 20 users
….
}
I try to get the above using the following mapReduce call:
db.user.mapReduce(
function(){var date = this._id.getTimestamp();
emit(new Date(date.getFullYear()+"-"+date.getMonth()+"-"+date.getDate()), 1)},
function(key, values) {cum = cum + Array.sum(values); return cum},
{out: "newUserAnalysis",
sort: {_id: 1},
scope: {cum: 0}})
But it seems that the cum variable reset to zero after the first return statement encountered in the reduce function. Why? Is there any other method to get what I want?
Many thanks.

Answer 1

cum should not be reset as it's a global variable in map, reduce and finalize functions during the whole mapReduce processing.
But reduce function has 3 requirements to be observed to assure processing correctly, particularly for bulky data handling since reduce function will be called repeatedly even on the same key. Normally the length of values in map function would not exceed 100. In a word, your design can't assure cum is called on the right sequence as you expect, which will produce incorrect statistics.
Following code for your reference:
// map and reduce per day then save to a collection.
db.user.mapReduce(function() {
var date = this._id.getTimestamp();
emit(new Date(date.getFullYear() + "-" + (date.getMonth() + 1) + "-"
+ date.getDate()), 1);
}, function(key, values) {
return Array.sum(values);
}, {
out : "newUserAnalysis",
sort : {
_id : 1
}
});
// Do accumulation one by one.
var cursor = db.newUserAnalysis.find().sort({_id:1});
var newValue = 0, first = true;
while (cursor.hasNext()) {
var doc = cursor.next();
newValue += doc.value;
if (first) {
first = false;
} else {
db.newUserAnalysis.update({_id:doc._id}, {$set:{value:newValue}});
}
}