define_callbacks

# File activerecord/lib/active_record/observer.rb, line 124
      def define_callbacks(klass)
        existing_methods = klass.instance_methods.map { |m| m.to_sym }
        observer = self
        observer_name = observer.class.name.underscore.gsub('/', '__')

        self.class.observed_methods.each do |method|
          callback = "_notify_#{observer_name}_for_#{method}""_notify_#{observer_name}_for_#{method}"
          unless existing_methods.include? callback
            klass.send(:define_method, callback) do  # def _notify_user_observer_for_before_save
              observer.update(method, self)          #   observer.update(:before_save, self)
            end                                      # end
            klass.send(method, callback)             # before_save :_notify_user_observer_for_before_save
          end
        end
      end