Immutable models in Ruby

model.rb

require 'set'

class Model
  def self.attributes *attrs
    @attributes ||= Set.new
    attr_reader *attrs
    @attributes += attrs
  end

  def initialize attributes={}
    attributes.each do |attr, value|
      instance_variable_set "@#{attr}", value
    end
  end

  def update new_attributes={}
    self.class.new(to_h.merge(new_attributes))
  end

  def to_h
    self.class.attributes.reduce({}) { |hash, attr|
      hash.merge! attr => send(attr)
    }
  end
end

user.rb

class User < Model
  attributes :name, :email
end