This probably rates coverage in the docs, since it's not obvious and I haven't seen anything published on the issue.
In fact, I'd consider it a bug except that I've seen similar behavior in Kodo JDO, so I can only assume I missed something here. The issue has to do with using the detach/attach behavior in a local web application context. This is an area that doesn't get much press - all the usual examples have to do with shipping out objects remotely, getting back the modified objects, and merging them. However, in some environments - notably JavaServer Faces - there is theoretically an advantage in working with detached objects that are stored in an HttpSession. A JSF conversation can typically present a form backed (directly or indirectly) by a datamodel object (forgoing a distinct DTO). That datamodel object can then be merged. So far well and good. The problem is, if that object - or the object returned from the merge - is then later merged again, an OptimisticLockingException results and the reasons aren't obvious. The cure, as it turns out, is to discard the results of the merge, re-fetch the object (e.g., by primary key) and then store that object in the session so that it can be presented for the next set of changes. Just to make things more interesting, the object returned from the merge and the object fetched are often the same object, except that the fetch appears to reset the "dirty" bits, which seems to be where the OptimisticLockingException comes from. Presumably, the overhead from a re-fetch is fairly low (cache hit), but since merge is supposedly returning an updated object, the need isn't obvious, so I'd like someone to tell me if that's the proper thing to do. The environment that works most naturally here (tiered architecture) has some quirks. Because of all the back-and-forth between the different levels of the JSF system, it's not really possible to attach and set up a transaction at the form level. Hence the need for a merge. The actual data operations are done in a transactionally-wrapped service layer method and the merge is done inside that method. Any insights, document references, etc. will be greatly appreciated.
