ActiveModel::ValueObject

example.rb

class Address
  include ActiveModel::ValueObject

  attribute :street
  attribute :city
end

class User < ActiveRecord::Base
  attr_accessible :address
  serialize :address, Address

  def address=(new_address)
    super Address(new_address)
  end
end

class Venue < ActiveRecord::Base
  attr_accessible :address
  serialize :address, Address

  def address=(new_address)
    super Address(new_address)
  end
end

user = User.new address: {city: 'NYC', street: '5th Ave'}
user.address.street # => "5th Ave"

user.address.city = 'LA' # Raises RuntimeError because object is frozen

venue = Venue.new
venue.address = Address.new city: 'NYC', street: 'Madison Ave'
user.address == venue.address # => false

venue.address = Address.new city: 'NYC', street: '5th Ave'
user.address == venue.address # => true

value_object.rb

module ActiveModel
  module ValueObject
    def self.included(base)
      base.class_eval do
        include ActiveModel::Model
        extend ClassMethods
        include InstanceMethods
      end

      Kernel.send :define_method, base.name do |params|
        base.new(base === params ? params.attributes : params)
      end
    end

    module ClassMethods
      def load(data)
        new JSON.load(data)
      end

      def dump(object)
        JSON.dump object.attributes
      end

      def attributes
        @attributes ||= []
      end

      def attribute(name)
        attributes << name
        attr_accessor name
      end
    end

    module InstanceMethods
      def initialize(params = {})
        super
        @attributes = self.class.attributes.each_with_object({}) { |attr, hash|
          hash[attr] = send(attr)
        }
        freeze
      end
    end

    attr_reader :attributes

    def to_s
      attrs_str = attributes.map { |attr, val| "#{attr}: #{val.inspect}"}.join(', ')
      "#{self.class.name}(#{attrs_str})"
    end

    def ==(other)
      attributes == other.attributes
    end
  end
end