fixing the list of cached columns

activerecord/test/cases/attribute_methods_test.rb

@@ -772,7 +772,9 @@ def title=(val); self.author_name = val; end
 
   private
   def cached_columns
-    @cached_columns ||= time_related_columns_on_topic.map(&:name)
+    Topic.columns.find_all { |column|
+      !Topic.serialized_attributes.include? column.name
+    }.map(&:name)
   end
 
   def time_related_columns_on_topic