diff --git a/activerecord/lib/active_record/dirty.rb b/activerecord/lib/active_record/dirty.rb
index 4ce0356457190..63bf8c8f5bac5 100644
--- a/activerecord/lib/active_record/dirty.rb
+++ b/activerecord/lib/active_record/dirty.rb
@@ -134,7 +134,9 @@ def write_attribute_with_dirty(attr, value)
 
       def update_with_dirty
         if partial_updates?
-          update_without_dirty(changed)
+          # Serialized attributes should always be written in case they've been
+          # changed in place.
+          update_without_dirty(changed | self.class.serialized_attributes.keys)
         else
           update_without_dirty
         end
diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index e5e022050dfc8..feb47a15a8eb6 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -191,6 +191,18 @@ def test_reload_should_clear_changed_attributes
     assert !pirate.changed?
   end
 
+  def test_save_should_store_serialized_attributes_even_with_partial_updates
+    with_partial_updates(Topic) do
+      topic = Topic.create!(:content => {:a => "a"})
+      topic.content[:b] = "b"
+      #assert topic.changed? # Known bug, will fail
+      topic.save!
+      assert_equal "b", topic.content[:b]
+      topic.reload
+      assert_equal "b", topic.content[:b]
+    end
+  end
+
   private
     def with_partial_updates(klass, on = true)
       old = klass.partial_updates?