instance method instantiate_observer

# File activemodel/lib/active_model/observing.rb, line 21
      def instantiate_observer(observer)
        # string/symbol
        if observer.respond_to?(:to_sym)
          observer = observer.to_s.camelize.constantize.instance
        elsif observer.respond_to?(:instance)
          observer.instance
        else
          raise ArgumentError, "#{observer} must be a lowercase, underscored class name (or an instance of the class itself) responding to the instance method. Example: Person.observers = :big_brother # calls BigBrother.instance"
        end
      end