Special case object

gistfile1.rb

class User
  def name
    # ...from the database
    "Fred"
  end

  def self.find(user_id)
    user_id == 123 ? self.new : nil
  end
end

class Guest
  def name
    "Anonymous"
  end
end

def current_user(session)
  User.find(session[:user_id]) || Guest.new
end

session = {}

session[:user_id] = 123
p current_user(session).name

session[:user_id] = 'NOT VALID'
p current_user(session).name