Merge pull request #6062 from marcandre/tweak_observer

Tweak observer

2 files changed, 21 insertions, 5 deletions

diff --git a/activemodel/lib/active_model/observing.rb b/activemodel/lib/active_model/observing.rb
index 35b1a1f0c7..cc60c9ac72 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
diff --git a/activemodel/test/cases/observing_test.rb b/activemodel/test/cases/observing_test.rb
index df070d2d8e..1a80b5d854 100644
--- a/activemodel/test/cases/observing_test.rb
+++ b/activemodel/test/cases/observing_test.rb
@@ -70,6 +70,21 @@ class ObservingTest < ActiveModel::TestCase
     ObservedModel.instantiate_observers
   end
 
+  test "raises an appropriate error when a developer accidentally adds the wrong class (i.e. Widget instead of WidgetObserver)" do
+    assert_raise ArgumentError do
+      ObservedModel.observers = ['string']
+      ObservedModel.instantiate_observers
+    end
+    assert_raise ArgumentError do
+      ObservedModel.observers = [:string]
+      ObservedModel.instantiate_observers
+    end
+    assert_raise ArgumentError do
+      ObservedModel.observers = [String]
+      ObservedModel.instantiate_observers
+    end
+  end
+
   test "passes observers to subclasses" do
     FooObserver.instance
     bar = Class.new(Foo)