association_ids.rb

module AssociationIds
  def ids_for name
    name = name.to_s
    define_method (name + '_ids=').to_sym do |ids|
      values = ids.collect { |id| Kernel.const_get(name.classify).find id }
      self.send(name + '=', values)
    end

    define_method (name + '_ids').to_sym do
      self.send(name).collect { |obj| obj.id }
    end
  end
end