conditional_validation_example_2.rb

class Movie < ActiveRecord::Base
  RATINGS = %w[G PG PG-13 R NC-17]
  validates_inclusion_of :rating, :in => RATINGS,
    :if => :after_ratings_adopted?

  def after_ratings_adopted?(movie)
    self.release_date >= Date.parse('1-Nov-1968')
  end

end