Merge pull request rails#6071 from marcandre/observer_redef

Fix Observer by acting on singleton class. Fixes rails#3505.

Author: jeremy

activemodel/lib/active_model/observing.rb

@@ -195,7 +195,7 @@ class << self
       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.

activemodel/test/cases/observing_test.rb

@@ -95,13 +95,13 @@ def setup
 class ObserverTest < ActiveModel::TestCase
   def setup
     ObservedModel.observers = :foo_observer
-    FooObserver.instance_eval do
+    FooObserver.singleton_class.instance_eval do
       alias_method :original_observed_classes, :observed_classes
     end
   end
 
   def teardown
-    FooObserver.instance_eval do
+    FooObserver.singleton_class.instance_eval do
       undef_method :observed_classes
       alias_method :observed_classes, :original_observed_classes
     end
@@ -160,4 +160,15 @@ def teardown
     end
     assert_equal :in_around_save, yielded_value
   end
+
+  test "observe redefines observed_classes class method" do
+    class BarObserver < ActiveModel::Observer
+      observe :foo
+    end
+
+    assert_equal [Foo], BarObserver.observed_classes
+
+    BarObserver.observe(ObservedModel)
+    assert_equal [ObservedModel], BarObserver.observed_classes
+  end
 end