MongoMapper MapReduce Example

score.rb

class Score
  include MongoMapper::Document
  
  key :user_id, ObjectId,   :index => true
  key :stage,   String,     :index => true
  key :score,   Integer
  timestamps!
  
  def self.high_score_map
    <<-MAP
      function() {
        emit(this.stage, { user_id : this.user_id, score: this.score });
      }
    MAP
  end
  
  def self.high_score_reduce
    <<-REDUCE
      function(k, v) {
        var top     = 0;
        var user_id = '';
        for (var i in v) {
          if (v[i]["score"] > top) {
            top     = v[i]["score"];
            user_id = v[i]["user_id"];
          }
        }
        return { user_id: user_id, score: top };
      }
    REDUCE
  end
  
  def self.high_score_build(opts)
    Score.collection.map_reduce(high_score_map, high_score_reduce, opts).find()
  end
  
  def self.high_score(opts={})
    data = []
    Score.high_score_build(opts).each do |score|
      data << { :user_id => score["value"]["user_id"], :stage => score["_id"], :score => score["value"]["score"] }
    end
    data
  end
  
  def self.high_score_daily(opts={})
    opts[:query][:created_at] = { "$gt" => Time.now - 1.day }
    Score.high_score(opts)
  end
  
  def self.high_score_weekly(opts={})
    opts[:query][:created_at] = { "$gt" => Time.now - 1.week }
    Score.high_score(opts)
  end
  
  def self.high_score_monthly(opts={})
    opts[:query][:created_at] = { "$gt" => Time.now - 1.month }
    Score.high_score(opts)
  end
  
end

class TopScore
  include MongoMapper::Document
  
  key :_id,   String
  key :value, Hash
  
  def self.compute_top_scores(opts={})
    opts[:out] = "top_scores"
    Score.high_score(opts)
  end

end

Modified version to handle the MapReduce output collection, and better options handling.

you can use the group method of mongodb from rails in a similar way?

I don't use Rails these days, and I don't think you would want to be sorting potentially millions of documents in Ruby code.

If you meant the mongodb group command exposed by a client driver, it didn't give the power to sort the way I wanted, thus the map reduce method.

It's too bad this code never made it to release, I was looking forward to seeing it in practice.