2 files changed, 18 insertions, 1 deletions
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 7ba907f786..0171ef3bdf 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -108,6 +108,7 @@ module ActiveRecord
       end
 
       def save_changed_attribute(attr, old_value)
+        clear_changed_attributes_cache
         if attribute_changed_by_setter?(attr)
           clear_attribute_changes(attr) unless _field_changed?(attr, old_value)
         else
@@ -176,7 +177,11 @@ module ActiveRecord
         @cached_changed_attributes = changed_attributes
         yield
       ensure
-        remove_instance_variable(:@cached_changed_attributes)
+        clear_changed_attributes_cache
+      end
+
+      def clear_changed_attributes_cache
+        remove_instance_variable(:@cached_changed_attributes) if defined?(@cached_changed_attributes)
       end
     end
   end
diff --git a/activerecord/lib/active_record/attribute_methods/serialization.rb b/activerecord/lib/active_record/attribute_methods/serialization.rb
index d0d8a968c5..e03bf5945d 100644
--- a/activerecord/lib/active_record/attribute_methods/serialization.rb
+++ b/activerecord/lib/active_record/attribute_methods/serialization.rb
@@ -11,6 +11,18 @@ module ActiveRecord
         # serialized object must be of that class on assignment and retrieval.
         # Otherwise <tt>SerializationTypeMismatch</tt> will be raised.
         #
+        # Empty objects as +{}+, in the case of +Hash+, or +[]+, in the case of
+        # +Array+, will always be persisted as null.
+        #
+        # Keep in mind that database adapters handle certain serialization tasks
+        # for you. For instance: +json+ and +jsonb+ types in PostgreSQL will be
+        # converted between JSON object/array syntax and Ruby +Hash+ or +Array+
+        # objects transparently. There is no need to use +serialize+ in this
+        # case.
+        #
+        # For more complex cases, such as conversion to or from your application
+        # domain objects, consider using the ActiveRecord::Attributes API.
+        #
         # ==== Parameters
         #
         # * +attr_name+ - The field name that should be serialized.