In your class, if you have a Collection of entities that you want to not have duplicates you can make it a Set, and be sure that you've provided a proper equals() and hashCode() for your entities; specifically, do not include the primary key because it starts out null when you instantiate the entity, and then is set after the entity is persisted. I don't know if there is anything on the JDO or DataNucleus site that explains this but I do remember reading about it in either a Hibernate book or on the Hibernate site.
tal wrote:
How do I enforce a unique constraint in GAE?
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