global variables in javascript / mongodb / mapreduce

Question

I know this has been asked countless times on the internet but I can't figure out what's going on here and I have been banging my head against walls for hours now.

This works :

in myscript.js :

obj = 'hello';
var f = function() {
  printjson('obj=' + obj);
}
f();

$ mongo myscript.js
obj=hello

This doesn't work :

date1 = "2013-09-03T00:00:00Z";
date2 = "2013-09-04T00:00:00Z";

var mapIntensities = function() {
  emit(this.unit, this.intensity);
};

var reduceIntensities = function(unit, intensities) {
  return {date: date1, "unit": unit, "intensity": Array.sum(intensities)};
};


db.units.mapReduce(mapIntensities, reduceIntensities, {out: "aggregate_intensities", query: {"DATE": {$gte: ISODate(date1), $lt: ISODate(date2)}}})

Why is that ? The problem occurs in the reduce() function (if just there I replace date1 with the hardcoded value it works)

Is it specific to mongodb's way to do mapreduce ? (as the working example suggests)

Sammaye · Accepted Answer · 2013-09-06 13:29:11Z

1

Because date1 is never defined within the Map Reduce itself, it is only ever used here:

return {date: date1, "unit": unit, "intensity": Array.sum(intensities)};

And then you do not define a variable to input at all into the actual call of the Map Reduce.

The Map Reduce runs within a self contained JavaScript (spidermonkey or v8) envo within MongoDB, not in the mongo console as I assume you are doing there.

You need to use the scope param: http://docs.mongodb.org/manual/reference/command/mapReduce/#dbcmd.mapReduce to send in date1

answered Sep 6, 2013 at 13:29

Sammaye

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