active-model-callbacks-1.rb

def define_model_callbacks(*callbacks)
  options = callbacks.extract_options!
  options = {
     :terminator => "result == false",
     :scope => [:kind, :name],
     :only => [:before, :around, :after]
  }.merge(options)

  types   = Array.wrap(options.delete(:only))

  callbacks.each do |callback|
    define_callbacks(callback, options)

    types.each do |type|
      send("_define_#{type}_model_callback", self, callback)
    end
  end
end

active-model-callbacks-2.rb

def _define_before_model_callback(klass, callback) #:nodoc:
  klass.class_eval <<-CALLBACK, __FILE__, __LINE__ + 1
    def self.before_#{callback}(*args, &block)
      set_callback(:#{callback}, :before, *args, &block)
    end
  CALLBACK
end

def _define_around_model_callback(klass, callback) #:nodoc:
  klass.class_eval <<-CALLBACK, __FILE__, __LINE__ + 1
    def self.around_#{callback}(*args, &block)
      set_callback(:#{callback}, :around, *args, &block)
    end
  CALLBACK
end

def _define_after_model_callback(klass, callback) #:nodoc:
  klass.class_eval <<-CALLBACK, __FILE__, __LINE__ + 1
    def self.after_#{callback}(*args, &block)
      options = args.extract_options!
      options[:prepend] = true
      options[:if] = Array.wrap(options[:if]) << "!halted && value != false"
      set_callback(:#{callback}, :after, *(args << options), &block)
    end
  CALLBACK
end

post.rb

class Post < ActiveRecord::Base
  before_save :generate_slug

  def generate_slug
    self.title = self.title.mb_chars.normalize(:kd).gsub(/[^\x00-\x7F]/n, '').to_s.gsub(/[']+/, '').gsub(/\W+/, ' ').strip.downcase.gsub!(' ', '-')
  end
end