Generate appropriate error more judiciously

2 files changed, 18 insertions, 2 deletions

diff --git a/activemodel/lib/active_model/observing.rb b/activemodel/lib/active_model/observing.rb
index 5aa27f00da..cc60c9ac72 100644
--- a/activemodel/lib/active_model/observing.rb
+++ b/activemodel/lib/active_model/observing.rb
@@ -90,8 +90,9 @@ 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,
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)