require 'active_support/lazy_load_hooks'
# ...(省略)...

# ActiveRecordが読み込まれたら以下のコードを実行する
# 第一引数(active_record)がフックのキー
# 第二引数のブロックが、ブロックを実行するためのコンテキスト
ActiveSupport.on_load(:active_record) do
  require 'switch_point/model'
  require 'switch_point/connection'

  # SwitchPoint::ModelのメソッドをActiveRecord::Baseにinclude
  ActiveRecord::Base.send(:include, SwitchPoint::Model)

  # class_evalで動的にクラス・メソッドを定義
  ActiveRecord::ConnectionAdapters::AbstractAdapter.class_eval do
    include SwitchPoint::Connection

    # insert, update, deleteメソッドのメソッドを上書き
    # 中ではwritableなDBにつなぎに行っているかのチェックとクエリー・キャッシュをクリアしてからメソッドを実行
    SwitchPoint::Connection::DESTRUCTIVE_METHODS.each do |method_name|
      alias_method_chain method_name, :switch_point
    end
  end
end

require 'active_support/lazy_load_hooks'

# run_load_hooksが実行されていないので呼ばれない
ActiveSupport.on_load :fuga do
  puts 'start 1st on load'
  fuga
  puts 'end 1st on load'
end

class Fuga
  def self.fuga
    puts 'fugafuga'
  end
end

puts 'before run_load_hooks'
# run_load_hooksを実行すると、4行目の ActiveSupport.on_load が呼ばれる
ActiveSupport.run_load_hooks :fuga, Fuga
puts 'after run_load_hooks'

# run_load_hooks が実行されているのですぐに呼ばれる
ActiveSupport.on_load :fuga do
  puts 'start 2st on load'
  fuga
  puts 'end 2st on load'
end

# 実行結果
# before run_load_hooks
# start 1st on load
# fugafuga
# end 1st on load
# after run_load_hooks
# start 2st on load
# fugafuga
# end 2st on load

# active_supportのメソッドなのでrequireが必要
require 'active_support/all'

module LogHelper

  def log(message)
    puts message
  end

  def log_with_timestamp(message)
    log_without_timestamp("[#{Time.now}] #{message}")
  end

  # 以下と同義
  # alias_method :log, :log_with_timestamp
  # alias_method :log_without_timestamp, :log
  alias_method_chain :log, :timestamp
end

include LogHelper
log('Hello') #=> [2015-01-02 11:45:01 +0900] Hello

  def alias_method_chain(target, feature)
    # Strip out punctuation on predicates, bang or writer methods since
    # e.g. target?_without_feature is not a valid method name.
    aliased_target, punctuation = target.to_s.sub(/([?!=])$/, ''), $1
    yield(aliased_target, punctuation) if block_given?

    with_method = "#{aliased_target}_with_#{feature}#{punctuation}"
    without_method = "#{aliased_target}_without_#{feature}#{punctuation}"

    alias_method without_method, target
    alias_method target, with_method

    case
    when public_method_defined?(without_method)
      public target
    when protected_method_defined?(without_method)
      protected target
    when private_method_defined?(without_method)
      private target
    end
  end

module LoggingWithTimestamp

  def log(message)
    # 呼び出し元のメソッドを呼び出す
    super("[#{Time.now}] #{message}")
  end

end

class Logging
  # LoggingWithTimestampのlogが、Loggingのlogメソッドよりも優先して呼ばれる
  prepend LoggingWithTimestamp

  def log(message)
    puts message
  end
end

Logging.new.log('Hello') #=> [2015-01-02 12:23:15 +0900] Hello