class ActiveRecord::Base

def self.object_id_attr(symbol, klass)
  module_eval %{
    def #{symbol}
      if @#{symbol}.nil? or (@old_#{symbol}_id != #{symbol}_id)
        @old_#{symbol}_id = #{symbol}_id
        klass = #{klass}.descendants.find { |d| d.#{symbol}_name == #{symbol}_id }
        klass ||= #{klass}
        @#{symbol} = klass.new
      else
        @#{symbol}
      end
    end
  }
end