observer_example.rb

class EmailList
  observe Review
  def after_create(review)
    moviegoers = review.moviegoers # from has_many :through, remember?
    self.email(moviegoers, "A new review for #{review.movie} is up.")
  end
  observe Moviegoer
  def after_create(moviegoer)
    self.email([moviegoer], "Welcome, #{moviegoer.name}!")
  end
  def self.email ; ... ; end
end