I was working on some map/reduce that rolled up daily, weekly and yearly statistics in MongoDB and discovered, to my surprise, that JavaScript Date doesn’t have a getWeek method. Worse, the piece of code on About.com turned out to be buggy (it has issues with week 1 and 52). Total Internet #fail. In this post I’ll show you how to add getWeek(date) to MongoDB/Mongoid and how to use it from a map/reduce.

The server-side JavaScript is almost like a stored procedure and is documented here. Lets use this implementation with a slight change in parameters and save it as lib/javascripts/getWeek.js. We can then store the JavaScript server-side in any Mongoid model. In our case we’ll be counting Widgets, so add this to Widget.rb.

1. def self.install_javascript
2.   getWeekJs = Rails.root.join("lib/javascript/getWeek.js")
3.   if collection.master['system.js'].find_one({'_id' => "getWeek"}).nil?
4.     collection.master.db.add_stored_function("getWeek", File.new(getWeekJs).read)
5.   end
6. end

The add_stored_function method comes from the Ruby MongoDB driver. Call Widget.install_javascript somewhere in a Rake task or inside your map/reduce code.

Lets now map/reduce our widgets into widgets_weekly using the created_at timestamp. Notice the call to getWeek.

1. def self.rollup_weekly
2.   map = <<-EOS
3.     function() {
4.         emit({'ts': this.created_at.getFullYear() + '-' + getWeek(this.created_at) }, {count: 1})
5.     }
6.   EOS
7.   reduce = <<-EOS
8.     function(key, values) {
9.       var count = 0;
10.       values.forEach(function(value) {
11.         count += value['count'];
12.       });
13.       return({ count: count });
14.     }
15.   EOS
16.   collection.map_reduce(map, reduce, :out => "widgets_weekly", :query => {})    
17. end

This yields the following collection in widgets_weekly.

1. { "_id" : { "ts" : "2011-1" }, "value" : { "count" : 73 } }
2. { "_id" : { "ts" : "2011-2" }, "value" : { "count" : 60 } }
3. { "_id" : { "ts" : "2011-3" }, "value" : { "count" : 31 } }
4. { "_id" : { "ts" : "2011-4" }, "value" : { "count" : 73 } }
5. { "_id" : { "ts" : "2011-5" }, "value" : { "count" : 32 } }

If anyone knows of a library that does this kind of rollups, OLAP cubes or any other data transformation for reporting purposes with MongoDB/Mongoid, please speak up!