mongo_mapper_order_by_array_ids.rb

require 'rubygems'
require 'mongo_mapper'

  MongoMapper.connection = Mongo::Connection.new('localhost',27017)
  MongoMapper.database = "test"

class Word
    include MongoMapper::Document
    key :content, String
    key :translation_ids, Array
    many :translations, :in => :translation_ids do
        def ordered_like_in_translation_ids
            sort_by {|a| proxy_owner.translation_ids.index(a.id)}
        end
    end
end

class Translation
    include MongoMapper::Document
    key :content, String
    key :lang, String
    def random
        rand
    end
end

Word.collection.remove

Translation.collection.remove

w = Word.new(:content => "1")
w.translations = [Translation.new(:content => "one",:lang=>"pl"), Translation.new(:content=>"two"), Translation.new(:content => "three",:lang=>"ja")]
w.save

puts Word.first.translations.map(&:content).inspect
puts "Lets make translation_ids reverse"
w=Word.first
w.translation_ids.reverse!;
w.save
w.reload
puts "order haven\'t changed"
puts Word.first.translations.map(&:content).inspect
puts "but when we use ordered_like_in_translation_ids method ...."