Revert "Revert "Handle enabling/disabling observers at different levels of the class hierarchy.""

This reverts commit 2a25c5818b03d7d6cd63aad180bff23479dbd861.

I'm going to add another commit that keeps the same behavior of fixes
the problems of leaking memory in development.

3 files changed, 75 insertions, 3 deletions

diff --git a/activemodel/lib/active_model/observer_array.rb b/activemodel/lib/active_model/observer_array.rb
index b8aa9cc1e2..f3b5811b81 100644
--- a/activemodel/lib/active_model/observer_array.rb
+++ b/activemodel/lib/active_model/observer_array.rb
@@ -48,7 +48,7 @@ module ActiveModel
       set_enablement(true, observers, &block)
     end
 
-    private
+    protected
 
       def observer_class_for(observer)
         return observer if observer.is_a?(Class)
@@ -61,13 +61,37 @@ module ActiveModel
         end
       end
 
+      def start_transaction
+        disabled_observer_stack.push(disabled_observers.dup)
+        each_subclass_array do |array|
+          array.start_transaction
+        end
+      end
+
+      def disabled_observer_stack
+        @disabled_observer_stack ||= []
+      end
+
+      def end_transaction
+        @disabled_observers = disabled_observer_stack.pop
+        each_subclass_array do |array|
+          array.end_transaction
+        end
+      end
+
       def transaction
-        orig_disabled_observers = disabled_observers.dup
+        start_transaction
 
         begin
           yield
         ensure
-          @disabled_observers = orig_disabled_observers
+          end_transaction
+        end
+      end
+
+      def each_subclass_array
+        model_class.subclasses.each do |subclass|
+          yield self.class.for(subclass)
         end
       end
 
@@ -92,6 +116,10 @@ module ActiveModel
               disabled_observers << klass
             end
           end
+
+          each_subclass_array do |array|
+            array.set_enablement(enabled, observers)
+          end
         end
       end
   end
diff --git a/activemodel/lib/active_model/observing.rb b/activemodel/lib/active_model/observing.rb
index e1a2ce218d..ba6be46670 100644
--- a/activemodel/lib/active_model/observing.rb
+++ b/activemodel/lib/active_model/observing.rb
@@ -70,6 +70,10 @@ module ActiveModel
         observer_instances.size
       end
 
+      def subclasses
+        @subclasses ||= []
+      end
+
       protected
         def instantiate_observer(observer) #:nodoc:
           # string/symbol
@@ -89,6 +93,7 @@ module ActiveModel
         # Notify observers when the observed class is subclassed.
         def inherited(subclass)
           super
+          subclasses << subclass
           notify_observers :observed_class_inherited, subclass
         end
     end
diff --git a/activemodel/test/cases/observer_array_test.rb b/activemodel/test/cases/observer_array_test.rb
index 215ca80bb4..38e4fd59fc 100644
--- a/activemodel/test/cases/observer_array_test.rb
+++ b/activemodel/test/cases/observer_array_test.rb
@@ -118,5 +118,44 @@ class ObserverArrayTest < ActiveModel::TestCase
       ORM.observers.disable Widget
     end
   end
+
+  test "allows #enable at the superclass level to override #disable at the subclass level when called last" do
+    Widget.observers.disable :all
+    ORM.observers.enable :all
+
+    assert_observer_notified Widget, WidgetObserver
+    assert_observer_notified Budget, BudgetObserver
+    assert_observer_notified Widget, AuditTrail
+    assert_observer_notified Budget, AuditTrail
+  end
+
+  test "allows #disable at the superclass level to override #enable at the subclass level when called last" do
+    Budget.observers.enable :audit_trail
+    ORM.observers.disable :audit_trail
+
+    assert_observer_notified     Widget, WidgetObserver
+    assert_observer_notified     Budget, BudgetObserver
+    assert_observer_not_notified Widget, AuditTrail
+    assert_observer_not_notified Budget, AuditTrail
+  end
+
+  test "can use the block form at different levels of the hierarchy" do
+    yielded = false
+    Widget.observers.disable :all
+
+    ORM.observers.enable :all do
+      yielded = true
+      assert_observer_notified Widget, WidgetObserver
+      assert_observer_notified Budget, BudgetObserver
+      assert_observer_notified Widget, AuditTrail
+      assert_observer_notified Budget, AuditTrail
+    end
+
+    assert yielded
+    assert_observer_not_notified Widget, WidgetObserver
+    assert_observer_notified     Budget, BudgetObserver
+    assert_observer_not_notified Widget, AuditTrail
+    assert_observer_notified     Budget, AuditTrail
+  end
 end