null objects pattern

null_book.rb

class NullBook
  def title
    'No book exists'
  end
end

order_class.rb

class Order
  def book
    @book
  end
end

order_view.rb

@orders.each do |order|
  order.book.title
end

order_view_patched.rb

@orders.each do |order|
  if @order.book.nil?
    'No Book exists'
  else
    order.book.title
  end
end

order_with_nullbook.rb

class Order
  def book
    @book || NullBook.new
  end
end