像这样一段代码：

require 'rubygems'
require 'active_support/all'

module A
  def f
    puts 'this is a'
  end
end

module B
  def f
    puts 'this is b'
    super
  end
end

class C
  include A
  include B
end

puts '** test 1, no hacking'
C.new.f

module A
  def f_with_test_a
    puts 'in HackingA'
    f_without_test_a
    puts 'out HackingA'
  end

  alias_method_chain :f, :test_a
end

puts '** test 2, hacking A'
C.new.f

module B
  def f_with_test_b
    puts 'in HackingB'
    f_without_test_b
    puts 'out HackingB'
  end

  alias_method_chain :f, :test_b
end

puts '** test 3, hacking both A & B'
C.new.f

这段代码测试 Ruby 在两层 alias_method 下的情况，这里创建了两个 module，并且用一个类去 include 它们，测试就是对两个 module 的同名方法分别做 alias_method 的 hack，看看是否确实能执行正常。

Ruby 1.9 中执行结果：

** test 1, no hacking
this is b
this is a
** test 2, hacking A
this is b
in HackingA
this is a
out HackingA
** test 3, hacking both A & B
in HackingB
this is b
in HackingA
this is a
out HackingA
out HackingB

可以看到结果是正确的

而 Ruby 1.8 下却是：

** test 1, no hacking
this is b
this is a
** test 2, hacking A
this is b
in HackingA
this is a
out HackingA
** test 3, hacking both A & B
in HackingB
this is b
1.rb:13:in `f_without_test_b': super: no superclass method `f' for #<C:0x10e5aaeb0> (NoMethodError)
    from 1.rb:41:in `f'
    from 1.rb:49

可以看到在 module B 执行到 super 的时候找不到祖先的 f 方法。。。