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Hibernate version:
Hibernate 3.0
Mapping documents:

Code between sessionFactory.openSession() and session.close():
Session session = SessionManager.createSessionFactory().openSession();
Connection conn = session.connection();

session.save(message);
Transaction tran = session.beginTransaction();
tran.commit();
Full stack trace of any exception that occurs:
[junit] 1) addMessagesTest(com.buffering.DatabaseSenderBufferTest)org.hibernate.HibernateException: could not determine type of dynamic entity
[junit] at org.hibernate.impl.SessionImpl.guessEntityName(SessionImpl.java:1472)
[junit] at org.hibernate.impl.SessionImpl.getEntityPersister(SessionImpl.java:1086)
[junit] at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:83)
[junit] at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:184)
[junit] at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
[junit] at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:173)
[junit] at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:27)
[junit] at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:69)
[junit] at org.hibernate.impl.SessionImpl.save(SessionImpl.java:481)
[junit] at org.hibernate.impl.SessionImpl.save(SessionImpl.java:476)
[junit] at com.buffering.DatabaseSenderBuffer.add(Unknown Source)
[junit] at com.buffering.DatabaseSenderBufferTest.addMessage(Unknown Source)
[junit] at com.buffering.DatabaseSenderBufferTest.addMessagesTest(Unknown Source)
[junit] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[junit] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
[junit] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
[junit] at com.buffering.DatabaseSenderBufferTest.main(Unknown Source)
[junit] FAILURES!!!
[junit] Tests run: 1, Failures: 0, Errors: 1
Name and version of the database you are using:
PostgreSQL 8.0.3

The generated SQL (show_sql=true):

Debug level Hibernate log excerpt:
---------------

I'm trying to save a Hashtable (message) wich extends from hashtable and an own interface. I have this "could not determine type of dynamic entity" Exception. By the way I can save other instances, from other classes without problems and even I can't save the Instance of message I can use the load method to recover instances from the database without problems.
I search Internet and this forums and I can't find a solution, any idea?¿
The mapping file is correct.