rails魔术字段的实现 ,alias_method_chain用法,for classmethod

Encapsulates the common pattern of:

  alias_method :foo_without_feature, :foo
  alias_method :foo, :foo_with_feature

With this, you simply do:

  alias_method_chain :foo, :feature
替我们定义了两个方法：foo_with_feature 和 foo_without_feature <保存原来的foo方法>
def foo_with_feature
  在此添加想向foo方法里面写的代码
  foo_without_feature 调用一下原来的方法
end


And both aliases are set up for you.

Query and bang methods (foo?, foo!) keep the same punctuation:

  alias_method_chain :foo?, :feature

is equivalent to

  alias_method :foo_without_feature?, :foo?
  alias_method :foo?, :foo_with_feature?

so you can safely chain foo, foo?, and foo! with the same feature.

module ActiveRecord
  # Active Record automatically timestamps create and update operations if the table has fields
  # named created_at/created_on or updated_at/updated_on.
  #
  # Timestamping can be turned off by setting
  #   <tt>ActiveRecord::Base.record_timestamps = false</tt>
  #
  # Timestamps are in the local timezone by default but can use UTC by setting
  #   <tt>ActiveRecord::Base.default_timezone = :utc</tt>
  module Timestamp
    def self.included(base) #:nodoc:
      base.alias_method_chain :create, :timestamps
      base.alias_method_chain :update, :timestamps
值得注意的是 :create , :update 这两个方法均为实例方法，它和类方法 :create ,:update 重名，但最终类方法也要去掉实例的:create 和 :update方法。

   这里并不是把实例方法用别名定义为类方法。   再说也不可能把实例方法定义为类方法
。
base.class_inheritable_accessor :record_timestamps, :instance_writer => false
      base.record_timestamps = true
    end

    private
      def create_with_timestamps #:nodoc:
        if record_timestamps
          t = self.class.default_timezone == :utc ? Time.now.utc : Time.now
          write_attribute('created_at', t) if respond_to?(:created_at) && created_at.nil?
          write_attribute('created_on', t) if respond_to?(:created_on) && created_on.nil?

          write_attribute('updated_at', t) if respond_to?(:updated_at)
          write_attribute('updated_on', t) if respond_to?(:updated_on)
        end
        create_without_timestamps
      end

      def update_with_timestamps #:nodoc:
        if record_timestamps
          t = self.class.default_timezone == :utc ? Time.now.utc : Time.now
          write_attribute('updated_at', t) if respond_to?(:updated_at)
          write_attribute('updated_on', t) if respond_to?(:updated_on)
        end
        update_without_timestamps
      end
  end
end

alias_method_chain  用于类方法
重新定义 ActiveRecord::Base.find方法，加上特定的查询

module FooBar

  def self.included(base)
    base.extend ClassMethods
    base.class_eval do
      class << self
        alias_method_chain :find, :creator
      end
    end
  end

  module ClassMethods

    def find_with_creator(*args)
      if self.columns.include?('creator')
         with_scope(:find => {:conditions=> [sql_str, value_hash] }) do
              find_without_creator(*args)
            end
         end
    else
       find_without_creator(*args)
    end

  end

end