Calculate a certain attribute only until the (active) record has been saved to the database.

calculate_until_saved.rb

module CalculateUntilSaved

  def self.included(base)
    base.extend(ClassMethods)
  end
  
  module ClassMethods
    def calculate_until_saved(attribute_name)
      
      attr_protected attribute_name
      
      before_validation :on => :create do
        self.send("#{attribute_name}=", send(attribute_name))
      end
      
      define_method("#{attribute_name}_with_reading_attribute") do
        attribute = read_attribute(attribute_name)
        return attribute if attribute
        send("#{attribute_name}_without_reading_attribute")
      end
      alias_method_chain attribute_name, :reading_attribute
      
    end
  end
  
end

usage.rb

class Booking < ActiveRecord::Base
  include CalculateUntilSaved

  # has an attribute named price
  def price
    # some really expensive or slow calculation
  end
  calculate_until_saved :price

end