Define single-instance lifecycle hooks for ActiveRecord 2 objects. Very useful if you want to do something after_save based on a dirty attribute observed before_save.

lifecycle_hooks.rb

module InstanceLifecycleHooks
  def self.included(base)
    base.module_eval do
      alias_method_chain :run_callbacks, :instance_lifecycle_hooks
    end
  end

  ActiveRecord::Callbacks::CALLBACKS.each do |callback|
    module_eval <<-RUBY, __FILE__, __LINE__+1
      def #{callback}_instance(method = nil, &block)
        @#{callback}_callback_chain ||= ActiveSupport::Callbacks::CallbackChain.new
        @#{callback}_callback_chain << ActiveSupport::Callbacks::Callback.new(#{callback.to_sym.inspect}, method || block, {})
      end
    RUBY
  end

  def run_callbacks_with_instance_lifecycle_hooks(kind, options = {}, &block)
    instance_variable_name = :"@#{kind}_callback_chain"
    if instance_variable_defined?(instance_variable_name)
      instance_chain = instance_variable_get(instance_variable_name)
      begin
        instance_chain.run(self, options, &block)
      ensure
        remove_instance_variable(instance_variable_name)
      end
    end
    run_callbacks_without_instance_lifecycle_hooks(kind, options, &block)
  end
end