dangjlin

module FirstLevelModule 
  def self.included(base)
    base.extend ClassMethods
    base.send :include, SecondLevelModule
  end

dangjlin

module SecondLevelModule 
  def self.included(base)
    base.extend ClassMethods
  end

  def second_level_instance_method; 'ok'; end

  module ClassMethods
    def second_level_class_method; 'ok'; end
  end 

dangjlin

module  ActiveRecord
  module Validations
    #...
    def self.included(base)
      base.extend ClassMethods
      #...
    end
    
    module ClassMethods
      def validates_length_of(*attrs)#...

dangjlin

require 'active_model' 
class User
  include ActiveModel::Validations
  attr_accessor :password
  validate do
  errors.add(:base, "Don't let dad choose the password.") if password == '1234'
  end 
end

dangjlin

module ActiveRecord
  # = Active Record Validations
  # Active Record includes the majority of its validations from <tt>ActiveModel::Validations</tt>
  # all of which accept the <tt>:on</tt> argument to define the context where the
  # validations are active. Active Record will always supply either the context of
  # <tt>:create</tt> or <tt>:update</tt> dependent on whether the model is a
  # <tt>new_record?</tt>.
  module Validations
    extend ActiveSupport::Concern
    include ActiveModel::Validations

dangjlin

module ActiveRecord 
    class Base
    extend ActiveModel::Naming
    extend ActiveSupport::Benchmarkable
...
...
    extend Translation
    extend DynamicMatchers
    extend Explain
    extend Enum

dangjlin

require "active_support/inflector/methods"

module ActiveSupport
  # Autoload and eager load conveniences for your library.
  #
  # This module allows you to define autoloads based on
  # Rails conventions (i.e. no need to define the path
  # it is automatically guessed based on the filename)
  # and also define a set of constants that needs to be
  # eager loaded:

dangjlin

#--
# Copyright (c) 2005-2015 David Heinemeier Hansson
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so, subject to
# the following conditions:

dangjlin

require 'active_support' 
require 'active_model'
# ...
module ActiveRecord
  extend ActiveSupport::Autoload
  autoload :Base 
  autoload :NoTouching 
  autoload :Persistence 
  autoload :QueryCache 
  autoload :Querying 

dangjlin

require 'active_record'
ActiveRecord::Base.establish_connection :adapter => "sqlite3",
                                        :database => "dbfile" 

# Initialize the database schema
ActiveRecord::Base.connection.create_table :ducks do |t|
  t.string  :name
end

class Duck < ActiveRecord::Base