Hwo to configure a JDO transaction to simulate creating a database sequence in App Engine Java?

IDs generated with IdGeneratorStrategy. SEQUENCE are unique for all entities with the same parent. If your entities are root entities (Eg, no parent), then they will all get unique IDs.

What is your use case where you have child entities, but need a unique ID across all of them?

In a question & answer site like this one, I want all questions to have a unique id across the app for use in the question url (stackoverflow. Com/questions/3817629/hwo-to-configure): The unique question id here is 3817629. The parent entity of the question is the user who writes the question but if the sequence is only unique-by-parent as you have said then there will be overlap of ids and they will not be usable for generating a unique url for the question.

Your explanation makes sense, though: I assumed the problem was with the JDO Fetch Groups when it's actually the Entity Groups. – Jimi Sep 29 '10 at 9:43 Why not just make the questions root entities, with a ReferenceProperty to the user that posted them, rather than making them child entities, then? – Nick Johnson Sep 29 '10 at 12:55 Not sure why I didn't think of that.

I just naturally thought of them as child entities. But I guess they don't need to be. Thanks.

– Jimi Sep 29 '10 at 13:20.

Whats wrong with IdGeneratorStrategy. SEQUENCE? Since GAE/J claims to support it.

Just as with IdGeneratorStrategy. IDENTITY, the sequence is unique within individual fetch groups but not across the application. – Jimi Sep 29 '10 at 7:58 I think you mean Entity Groups (a GAE/J BigTable concept).

A FetchGroup is a group of fields used when retrieving using JDO. If their sequence is not unique across the app then request them to fix it so it is (or at least is an option) – DataNucleus Sep 29 '10 at 9:10 Yes, you're right. I can see after reading Nick Johnson's response that I misunderstood the source of the problem and it is actually an Entity Group issue and not a Fetch Group issue.At least that's progress of sorts.

– Jimi Sep 29 '10 at 9:46.

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