A MongoMapper Model, the LocalizedString custom data type, and the Console Log.

Console Log

>> f = Foo.new({:title => "Title"})
NoMethodError: undefined method `translations' for "Title":String
	from /Volumes/MEDIA/projects/oratsouyts/lib/LocalizedString.rb:19:in `to_mongo'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/keys/key.rb:51:in `set'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/keys.rb:310:in `write_key'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/dirty.rb:59:in `write_key'
	from (eval):10:in `title='
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/keys.rb:183:in `send'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/keys.rb:183:in `attributes='
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/keys.rb:181:in `each_pair'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/keys.rb:181:in `attributes='
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/keys.rb:211:in `assign'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/accessible.rb:21:in `assign'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/protected.rb:30:in `assign'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/keys.rb:165:in `initialize'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/dirty.rb:14:in `initialize'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/sci.rb:26:in `initialize'
	from /Library/Ruby/Gems/1.8/bundler/gems/mongomapper-b3ab248dd907/lib/mongo_mapper/plugins/callbacks.rb:7:in `initialize'
	from (irb):1:in `new'
	from (irb):1>> 

foo.rb

class Foo
  include MongoMapper::Document

  key :title, LocalizedString
	
  timestamps!
end

LocalizedString.rb

class LocalizedString
  attr_accessor :translations

  def initialize( translations = {} ) 
    @translations = translations
  end 

  def self.from_mongo(value)
    if value.is_a?(Hash)
      LocalizedString.new(value)
    elsif value.nil?
      LocalizedString.new()
    else
      LocalizedString.new( { I18n.locale.to_s => value })
    end
  end

  def self.to_mongo(value)
    value.translations if value.present?
  end

end