buy.rb

class Buy
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::MongoTranslation


  has_many_related :criterias

  field :contact_id, :type => Integer, :index => true
  field :offer_type_id, :type => Integer, :index => true
  field :created_by_user_id, :type => Integer, :index => true

  field :status_id, :type => Integer, :index => true
  field :number, :type => Integer, :index => true
  field :user_id, :type => Integer, :index => true
  
  field :name
  field :description
  translates :name
  
  def user
    @user ||= User.find(user_id)
  end

end

gistfile2.txt

>> Buy.new
from -> Mongoid::Document.self.included(base = Buy)
from -> Mongoid::Fields.self.included(base = Buy)
from -> Mongoid::Document.self.included(base = #<Class:0xb60a4f40>)
from -> Mongoid::Fields.self.included(base = #<Class:0xb60a4f40>)
belongs_to buy :inverse_of => buy_translations

true
defining has_many buy_translations
initilize nil
=> #<Buy _id: 4bb507479ec2255dcd000001, name: nil, number: nil, created_at: nil, updated_at: nil, contact_id: nil, user_id: nil, offer_type_id: nil, description: nil, status_id: nil, created_by_user_id: nil>
>> BuyTranslation.new
initilize nil
NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.inject
	from /usr/lib/ruby/gems/1.8/gems/mongoid-1.2.14/lib/mongoid/fields.rb:38:in `defaults'
	from /usr/lib/ruby/gems/1.8/gems/mongoid-1.2.14/lib/mongoid/fields.rb:15:in `__send__'
	from /usr/lib/ruby/gems/1.8/gems/mongoid-1.2.14/lib/mongoid/fields.rb:15:in `defaults'
	from /usr/lib/ruby/gems/1.8/gems/mongoid-1.2.14/lib/mongoid/document.rb:277:in `attributes_with_defaults'
	from /usr/lib/ruby/gems/1.8/gems/mongoid-1.2.14/lib/mongoid/document.rb:168:in `initialize'
	from (irb):2:in `new'
	from (irb):2
>> 

mongo_translation.rb

# encoding: utf-8
module Mongoid
  module MongoTranslation
    def self.included(klass)
      klass.extend ClassMethods
    end

    def create_or_update_translations(params)
      translations = send("#{self.class.to_s}Translation".underscore.pluralize.to_sym)
      translation_class = Kernel.const_get("#{self.class.to_s}Translation")

      # update existing translations
      translations.each do |translation|
        translation.update_attributes(params.delete(translation.locale))
      end

      # create new translations
      (AVAILABLE_LOCALES - [I18n.default_locale]).each do |lang|
        attrs = params[lang.to_s]
        if attrs
          puts "update translation_class.new(attrs) = #{translation_class}.new(#{attrs.inspect})"
          tr = translation_class.new(attrs)
          tr.locale = lang.to_s
          translations << tr
        end
      end

      save
    end

    module ClassMethods

      def locale=(locale)
        @@locale = locale
      end

      def locale
        (defined?(@@locale) && @@locale) || I18n.locale
      end



      def translates(*fields)
        klass_name = "#{name.camelcase}Translation"
        parent_association = name.underscore.to_sym
        association = klass_name.underscore.pluralize.to_sym
        
        # Create the translation model
        translation_model = Class.new do
          include Mongoid::Document

          field :locale, :type => String
          fields.each do |field_name|
            field field_name.to_sym, :type => String
          end

          puts "belongs_to #{parent_association} :inverse_of => #{association}"
          belongs_to parent_association, :inverse_of => association

          puts collection_name
          puts embedded
        end
        Object.const_set klass_name, translation_model


        instance_eval do
          puts "defining has_many #{klass_name.underscore.pluralize.to_sym}"
          has_many klass_name.underscore.pluralize.to_sym
        end

        # Add attributes readers
        class_eval do
          fields.each do |field_name|

            define_method(field_name) do | *params |
              requested_locale = params.first
              requested_locale = I18n.locale if requested_locale.nil?
              if requested_locale == I18n.default_locale
                instance_variable_get("@#{field_name}")
              else
                translations = send(klass_name.underscore.pluralize.to_sym)
                translated_attr = nil
                translations.each do |translation|
                  translated_attr = translation.send(field_name.to_sym) if translation.locale.to_sym == requested_locale
                end
                translated_attr = instance_variable_get("@#{field_name}") if translated_attr.nil?
                translated_attr
              end
            end # define method
            
          end
        end
      end
    end
  end
end

__END__

b = Buy.new
I18n.locale = :bg
b.name ="ime"
I18n.locale = :ru
b.name ="poziwna"
b.inspect

translations = {
  'ru' => { 'name' => 'testNL'},
  'bg' => { 'name' => 'testEN'}
}

b.create_or_update_translations(translations)

I18n.locale = :bg
b.name
I18n.locale = :ru
b.name
b.inspect