1 files changed, 27 insertions, 19 deletions

diff --git a/activemodel/lib/active_model/observing.rb b/activemodel/lib/active_model/observing.rb
index 35b1a1f0c7..f5ea285ccb 100644
--- a/activemodel/lib/active_model/observing.rb
+++ b/activemodel/lib/active_model/observing.rb
@@ -90,14 +90,15 @@ module ActiveModel
         def instantiate_observer(observer) #:nodoc:
           # string/symbol
           if observer.respond_to?(:to_sym)
-            observer.to_s.camelize.constantize.instance
-          elsif observer.respond_to?(:instance)
+            observer = observer.to_s.camelize.constantize
+          end
+          if 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 " +
+              "#{observer} must be a lowercase, underscored class name (or " +
+              "the class itself) responding to the method :instance. " +
+              "Example: Person.observers = :big_brother # calls " +
               "BigBrother.instance"
           end
         end
@@ -109,17 +110,24 @@ module ActiveModel
         end
     end
 
-    private
-      # Fires notifications to model's observers
-      #
-      # def save
-      #   notify_observers(:before_save)
-      #   ...
-      #   notify_observers(:after_save)
-      # end
-      def notify_observers(method)
-        self.class.notify_observers(method, self)
-      end
+    # Fires notifications to model's observers
+    #
+    #   def save
+    #     notify_observers(:before_save)
+    #     ...
+    #     notify_observers(:after_save)
+    #   end
+    #
+    # Custom notifications can be sent in a similar fashion:
+    #
+    #   notify_observers(:custom_notification, :foo)
+    #
+    # This will call +custom_notification+, passing as arguments
+    # the current object and :foo.
+    #
+    def notify_observers(method, *extra_args)
+      self.class.notify_observers(method, self, *extra_args)
+    end
   end
 
   # == Active Model Observers
@@ -194,7 +202,7 @@ module ActiveModel
       def observe(*models)
         models.flatten!
         models.collect! { |model| model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model }
-        redefine_method(:observed_classes) { models }
+        singleton_class.redefine_method(:observed_classes) { models }
       end
 
       # Returns an array of Classes to observe.
@@ -229,10 +237,10 @@ module ActiveModel
 
     # Send observed_method(object) if the method exists and
     # the observer is enabled for the given object's class.
-    def update(observed_method, object, &block) #:nodoc:
+    def update(observed_method, object, *extra_args, &block) #:nodoc:
       return unless respond_to?(observed_method)
       return if disabled_for?(object)
-      send(observed_method, object, &block)
+      send(observed_method, object, *extra_args, &block)
     end
 
     # Special method sent by the observed class when it is inherited.