看下面一个场景:

# product.rb
class Product < ActiveRecord::Base
  validates_presence_of :price

  def self.find_ordered
    find(:all, : order => 'name')
  end
end

# product_test.rb
require File.dirname(__FILE__) + '/../test_helper'

class ProductTest < Test::Unit::TestCase
  def test_find_ordered_should_order_products_by_name
    Product.delete_all
    basket = Product.create!(:name => 'Basket')
    apple = Product.create!(:name => 'Apple')
    assert_equal [apple, basket], Product.find_ordered
  end
end

运行ProductTest，结果出错，因为Product的price有validates_presence_of声明

我们能否暂时将Product的validation关闭呢，比如:

# product_test.rb
require File.dirname(__FILE__) + '/../test_helper'

class ProductTest < Test::Unit::TestCase
  def test_find_ordered_should_order_products_by_name
    disable_validation do
      Product.delete_all
      basket = Product.create!(:name => 'Basket')
      apple = Product.create!(:name => 'Apple')
      assert_equal [apple, basket], Product.find_ordered
    end
  end
end

好，就让我们一起来打开ActiveRecord的盒子，Hack一把:

# test_helper.rb
class Test::Unit::TestCase
  self.use_transactional_fixtures = true
  self.use_instantiated_fixtures  = false

  def disable_validation
    ActiveRecord::Base.disable_validation!
    yield
    ActiveRecord::Base.enable_validation!
  end
end

module ValidationDisabler
  def self.included(base)
    base.class_eval do
      extend ClassMethods
      alias_method_chain :valid?, :disable_check
    end
  end

  def valid_with_disable_check?
    if self.class.validation_disabled?
      true
    else
      valid_without_disable_check?
    end
  end

  module ClassMethods
    def disable_validation!
      @@disable_validation = true
    end

    def enable_validation!
      @@disable_validation = false
    end

    def validation_disabled?
      @@disable_validation ||= false
    end
  end
end

class ActiveRecord::Base
  include ValidationDisabler
end

我们定义了disable_validation方法，先关闭validation，然后将控制权交给block执行，然后恢复validation

而ValidationDisabler模块的代码简直是优美无比!

我们首先定义self.included(base)方法
这个方法就像一个钩子，当下面我们打开ActiveRecord::Base类让它将ValidationDisabler include进来时触发
然后反过来让ActiveRecord::Base执行class_eval，让Base继承ValidationDisabler里的ClassMethods模块
并且给Base的valid?方法加上了AOP -- alias_method_chain

valid_with_disable_check?先判断validation_disabled?，如果返回true(即validation被disable掉了)，则valid?方法返回true
如果validation_disabled?返回false(即validation没有disable)，则调用valid_without_disable_check?(正常执行valid?方法)

ClassMethods模块里定义一个类变量@@disable_validation来决定是否关闭当前validation

如果大家留意Rails源码和许多Rails插件的源码的话，就会发现很多都是基于上述模式打开ActiveRecord、ActionController
等模块来修改Rails默认的行为的