conditional_validation_example.rb

class Movie < ActiveRecord::Base
  RATINGS = %w[G PG PG-13 R NC-17]  #  %w[] shortcut: arrays of strings
  @@grandfathered = Date.parse('1-Nov-1968')
  validates_inclusion_of :rating, :in => RATINGS,
    :if => lambda { |m| m.release_date >= @@grandfathered }
  # another way:
  validates_inclusion_of :rating, :in => RATINGS, :unless => :grandfathered
  def grandfathered ; release_date <= @@grandfathered ; end
end