如何在Appengine中保存数据存储实体的版本历史

I'm storing an Entity A in my datastore on appengine. A has an id of type Long. I'd like to keep a history of all changes made to A's fields. What are the best practices to doing this type of version ing on entities? I'd prefer a solution that works well with subclasses of A and is as automatic as possible.I'm storing an Entity A in my datastore on appe