Prevent callbacks in child classes from being executed more than once.

activesupport/lib/active_support/callbacks.rb

@@ -416,7 +416,7 @@ def __update_callbacks(name, filters = [], block = nil) #:nodoc:
         options = filters.last.is_a?(Hash) ? filters.pop : {}
         filters.unshift(block) if block
 
-        ([self] + ActiveSupport::DescendantsTracker.descendants(self)).each do |target|
+        ([self] + ActiveSupport::DescendantsTracker.descendants(self)).reverse.each do |target|
           chain = target.send("_#{name}_callbacks")
           yield target, chain.dup, type, filters, options
           target.__define_runner(name)

activesupport/test/callback_inheritance_test.rb

@@ -82,6 +82,30 @@ def do_nothing
   end
 end
 
+class CountingParent
+  include ActiveSupport::Callbacks
+
+  attr_reader :count
+
+  define_callbacks :dispatch
+
+  def initialize
+    @count = 0
+  end
+
+  def count!
+    @count += 1
+  end
+
+  def dispatch
+    run_callbacks(:dispatch)
+    self
+  end
+end
+
+class CountingChild < CountingParent
+end
+
 class BasicCallbacksTest < Test::Unit::TestCase
   def setup
     @index    = GrandParent.new("index").dispatch
@@ -147,4 +171,10 @@ def test_callbacks_looks_to_the_superclass_before_running
     child = EmptyChild.new.dispatch
     assert child.performed?
   end
+
+  def test_callbacks_should_be_performed_once_in_child_class
+    CountingParent.set_callback(:dispatch, :before) { count! }
+    child = CountingChild.new.dispatch
+    assert_equal 1, child.count
+  end
 end