Instantiate the results of map-reduce operations as Mongoid documents

criteria.rb

module Mongoid
  class Criteria
    include Criterion::MapReduce
  end
end

criterion.rb

module Mongoid
  module Criterion
    module MapReduce
      def map_reduce(map, reduce)
        options = self.options.dup
        options.merge!(:query => selector) if selector.present?
        klass.collection.map_reduce(map, reduce, options)
      end
    end
  end
end

map_reduce_document.rb

module Mongoid
  module MapReduceDocument
    extend ActiveSupport::Concern

    include Document #XXX Ideally we'd have a read-only document here.

    included do
      private_class_method :new
      Mongoid::Persistence.instance_methods.each do |method|
        undef_method method
      end
    end

    module ClassMethods
      def run(scope)
        collection = scope.all.map_reduce(@map, @reduce)
        clazz = Class.new(self)
        clazz.collection_name = collection.name
        clazz._collection = Mongoid::Collection.new(clazz, collection.name)
        clazz.create_indexes
        clazz.all
      end

      def instantiate(attrs)
        super(attrs['value'].merge('_id' => attrs['_id']))
      end

      def map(map)
        @map = map
      end

      def reduce(reduce)
        @reduce = reduce
      end

      def hereditary?
        !!(MapReduceDocument > superclass.superclass)
      end
    end
  end
end