Ruby Class mixin to create an accessor for which historical vales are kept in an Array, not thread-safe.

attr_history.rb

module AttrHistory
  def attr_history(name)
    attr_reader "#{name}_history", name

    define_method "#{name}=" do |value|
      history = send("#{name}_history") || []
      instance_variable_set "@#{name}_history", history.push(value)
      instance_variable_set "@#{name}", value
    end
  end
end

Class.send :include, AttrHistory