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Collection owner not associated with session? Not quite.

I hate when this happens. I upgraded to NHibernate 2.0 and then quickly afterward to 2.1.0 (you guessed it: because of LINQ). I had to change a couple of things to support it in my company’s application framework and it all seemed to work well – until I discovered that deleting any entity that has a one-to-many relation with cascade=”all-delete-orphan” stopped functioning. It died with a cryptic error message of “collection owner not associated with session”… If I changed to cascade=”all” it worked, but this is not the point, it wasn’t broken earlier. Of course, I tried looking all over the web and apart from a page in Spanish (which wouldn’t be helpful even if it was in English) came up blank. Tried moving to NHibernate 2.1.2 - which is not that simple since we’re using a slightly modified version of NHibernate (a reason more to suspect that the solution to this problem would be hard to find). So here’s a short post for anyone stumbling upon a similar problem.

In the end, I traced it to this behaviour: the collection owner is not found in the session because NHibernate tries to find it using ID = 0, while it’s original ID was 48. The logic is somewhat strange here, because the method receives the original collection owner (which is in the session), retrieves its ID (which was for some reason reset to 0) and then tries to find it using this wrong ID. Moreover, there’s a commented-out code that says “// TODO NH Different behavior” that would seem to do things properly (I checked it, it’s still standing in the NHibernate trunk as is). But the real reason why this happened is that blasted zero in the ID: further debugging (thankfully, there’s a full source for NHibernate available), revealed that it was reset because “use_identifier_rollback” was turned on in the configuration. Well… I probably set this to experiment with it and forgot. Turning it off solved the problem for me… Luckily, I didn’t really need this rollback functionality - as it’s not exactly what it seems to be: it doesn’t rollback identifiers when the transaction is rolled back, it rolls them back when entities are deleted! Why the second feature made more sense to implement than the first one is a mystery to me...

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