(暂)通常是因为不适当的flush() 引起异常的 。。。
When persisting a transient entity during manual flush, an exception
"javax.persistence.TransactionRequiredException: no transaction is in
progress" is thrown, unless getEntityManager().joinTransaction() is
executed prior to flush() - despite declaring the POJO as
@Transactional and using Seam-managed transactions and persistence
contexts.
When persisting a managed entity, however, no exceptions are thrown and update completes normally.