Fix Observer by acting on singleton class [#3505]

Also [issue #1034] [pull #6068]

2 files changed, 14 insertions, 3 deletions

diff --git a/activemodel/lib/active_model/observing.rb b/activemodel/lib/active_model/observing.rb
index 35b1a1f0c7..e303425a7c 100644
--- a/activemodel/lib/active_model/observing.rb
+++ b/activemodel/lib/active_model/observing.rb
@@ -194,7 +194,7 @@ 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.
diff --git a/activemodel/test/cases/observing_test.rb b/activemodel/test/cases/observing_test.rb
index df070d2d8e..096a309335 100644
--- a/activemodel/test/cases/observing_test.rb
+++ b/activemodel/test/cases/observing_test.rb
@@ -80,13 +80,13 @@ end
 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
@@ -145,4 +145,15 @@ class ObserverTest < ActiveModel::TestCase
     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