module DefaultValues

  def has_default_values(default_values = {})

    cattr_accessor :default_values
    self.default_values = default_values

    after_initialize :assign_default_values

    include InstanceMethods
  end

  module InstanceMethods
    private

    def assign_default_values
      return unless new_record?
      default_values.each do |key, value|
        self[key] = value if self[key].nil?
      end
    end
  end

end