Fix observer callbacks firing multiple times on descendant instances

activerecord/lib/active_record/observer.rb

@@ -108,10 +108,17 @@ def add_observer!(klass)
 
       def define_callbacks(klass)
         observer = self
+        observer_name = observer.class.name.underscore.gsub('/', '__')
 
         ActiveRecord::Callbacks::CALLBACKS.each do |callback|
           next unless respond_to?(callback)
-          klass.send(callback){|record| observer.send(callback, record)}
+          callback_meth = :"_notify_#{observer_name}_for_#{callback}"
+          unless klass.respond_to?(callback_meth)
+            klass.send(:define_method, callback_meth) do
+              observer.send(callback, self)
+            end
+            klass.send(callback, callback_meth)
+          end
         end
       end
   end

activerecord/test/cases/lifecycle_test.rb

@@ -7,6 +7,12 @@
 
 class SpecialDeveloper < Developer; end
 
+class DeveloperObserver < ActiveRecord::Observer
+  def before_save(developer)
+    developer.salary += 1
+  end
+end
+
 class SalaryChecker < ActiveRecord::Observer
   observe :special_developer
   attr_accessor :last_saved
@@ -195,4 +201,12 @@ def test_invalid_observer
     assert_equal developer, SalaryChecker.instance.last_saved
   end
 
+  test "callback observing the ancestor does not fire multiple times on descendent" do
+    DeveloperObserver.instance # activate
+    developer = Developer.create! :name => 'Ancestor', :salary => 100000
+    assert_equal 100001, developer.salary, 'ancestor callback fired multiple times'
+    developer = SpecialDeveloper.create! :name => 'Descendent', :salary => 100000
+    assert_equal 100001, developer.salary, 'descendent callback fired multiple times'
+  end
+
 end