gistfile1.rb

class VisitorSession
  include Mongoid::Document

  field :visits, :type => Integer, :default => 1
  field :project_id
  
  def visits_for_project
    map = <<EOF
      function() {
        if (this.project_id == '#{project_id}') {
          emit(this.visits, {visits: this.visits})
        }
      }
EOF

    reduce = <<EOF
      function(key, values) {
        var sum = 0;
        values.forEach(function(doc) {
          sum += doc.visits;
        });
        return {visits: sum};
      };
EOF

    collection.mapreduce(map, reduce).find().first["value"]["visits"].to_i 
  end
end