Paul Ingles

The test:

def test_cannot_change_country_name_once_constructed
  country = Country.new(:name => 'UK')
  assert_raise RuntimeError do
    country.name = 'USA'
  end
end

The class:

class Country < ActiveRecord::Base
  extend ImmutableModel
  immutable :name
  ...

The implementation:

module ImmutableModel
  def immutable(attr_name)
    define_method "#{attr_name}=" do |new_value|
      read_attribute(attr_name).nil? ? write_attribute(attr_name, new_value) : raise(RuntimeError, "You can't change #{attr_name} once it has been set")
    end
  end
end