Because you're using hibernate in update mode. It rebuilds all the tables and foreing keys .
You can cancel the update mode by removing
<property name="hibernate.hbm2ddl.auto" value="update"/>
line from the persistence.xml
Thank you. I'll try it ASAP.
Side effects of deleting that line from persistence.xml?
This line tells hibernate engine to rebuild all the database objects according to the entities . So if you don't need this, than no problem.:)