replace superclass temporally in Ruby

richdad.rb

require './surrogate_dad'

# your real
class PoorDad
  private
  def money
    100000
  end
end

class Me < PoorDad
  def give_me_money
    money * 0.01
  end
end

Me.new.give_me_money # => 1000.0


# Masashi's real
class RichDad
  private
  def money
    100000 * FaceBook.stock_value
  end
end

class FaceBook
  def self.stock_value
    38
  end
end

class Masashi < RichDad
  def give_me_money
    money * 0.01
  end
end

Masashi.new.give_me_money # => 38000.0


# now, Masashi's father becomes yours temporally!
include SurrogateDad

surrogate(Me, RichDad) do
  Me.superclass # => RichDad
  Me.class_eval do
    def give_me_money
      money * 0.01
    end
  end
  Me.new.give_me_money # => 38000.0
end

Me.new.give_me_money # => 1000.0

surrogate_dad.rb

module SurrogateDad
  def surrogate(klass, new_dad)
    class_name = :"#{klass}"
    real_class_name = :"Real#{klass}"
    self.class.class_eval do
      const_set(real_class_name, klass)
      remove_const(class_name)
      c = Class.new(new_dad)
      const_set(class_name, c)
    end
    yield
  ensure
    self.class.class_eval do
      remove_const(class_name)
      const_set(class_name, const_get(real_class_name))
      remove_const(real_class_name)
    end
  end
end