In Ruby, should I use ||= or if defined? for memoization?

Question

Should I use if defined?

 return @current_user_session if defined?(@current_user_session)
 @current_user_session = UserSession.find

Answer 1

Rails does have memoization, check out the screencast below for a great introduction:

http://railscasts.com/episodes/137-memoization

class Product < ActiveRecord::Base
  extend ActiveSupport::Memoizable

  belongs_to :category

  def filesize(num = 1)
    # some expensive operation
    sleep 2
    12345789 * num
  end

  memoize :filesize
end