Liz Douglass

Map Reduce in Mongo

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Recently I was interested in finding out which of our users has received the most coupons. We store information about each coupon in a MongoDB document that looks something like this:
    "_id" : ObjectId("4cb314e7c42af81a30ec3938"),
    "recipient" : {
        "$ref" : "users",
        "$id" : ObjectId("4cb314c7c42af81a238c3838")
    "text" : "Foo coupon",
    "dateTime" : "Wed May 12 2010 00:00:00 GMT+0100 (GMT)",

I started off looking at the Aggregation page of the MongoDB help documents. This page lists a number of querying options:

  • Count – useful if I wanted to know the number of coupons received by just one member, hence not quite what I was after
  • Distinct – for a whole other category of problems
  • Group – this looked exactly like what I needed, at least until I read the fine print: “Note: the result is returned as a single BSON object and for this reason must be fairly small – less than 10,000 keys, else you will get an exception. For larger grouping operations without limits, please use map/reduce.” We have 631,000 coupon documents. So map/reduce it is…

Following the example given here, I ended up with these functions:

m = function(){ emit(this.recipient.$id, {coupons : 1})};

r = function (key, values) {
    var sum = 0;
    for (var i = 0; i < values.length; i++) {
        sum += 1;
    return { coupons: sum };

Running this:

res =,r);

And then querying the results:

db[res.result].find().sort({"" : -1})

Gave the answer I needed:

{ "_id" : ObjectId("4cb314c5c42af81aa14f3838"), "value" : { "coupons" : 2236 } }
{ "_id" : ObjectId("4cb314c8c42af81adbb13838"), "value" : { "coupons" : 1133 } }
{ "_id" : ObjectId("4cb314c7c42af81ab2923838"), "value" : { "coupons" : 782 } }

This is a fairly straight forward map/reduce and was quite a nice introduction to them.

Written by lizdouglass

December 15, 2010 at 9:23 am

Posted in Uncategorized

Tagged with ,

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