Hello Markus,
I'm not sure if you are doing the persist in a separate application from the find, but if its the same one, you have to be sure to set all bidirectional relationships. If not, you will get problems such as this where the cache does not reflect what is in the database - this is required by the JPA specification.
TopLink Essentails has issolated and shared caches which you can configure to meet your usage needs. I'd strongly recommend you read Wonseok's blog on the issue at
http://weblogs.java.net/blog/guruwons/archive/2006/09/understanding_t_1.html
Regards,
Chris
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