метод сохранения - не сбрасывает сеанс после возникновения исключения

public class SoftwareTest extends UnitTest {

    @Before
    public void setup() {
        Fixtures.deleteAll(); // will fail if comment that. why?????
    }

    @Test
    public void createSoftwareWithNullAuthor()  {

       // when author is null

       Author nullAuthor = null;

       Software software = new Software("software1", "description1", nullAuthor);
       try {
         software.save();
         fail("author should not be null");
       } catch (PersistenceException ex) {
       }

    }


    @Test
    public void createSoftwareWithOkAuthor()  {
       // when author is ok
       Author okAuthor = new Author("author1", "email1").save(); // ERROR HERE!

       Software software2 = new Software("software2", "description2", okAuthor);
       Software savedSoftware = software2.save();
       assertNotNull(savedSoftware);
       assertEquals(savedSoftware, software2); 

       assertNotNull(savedSoftware.author);
       assertEquals(okAuthor, savedSoftware.author);
    }
}

раскомментируя строку с помощью Fixtures.deleteAll () , мы получим исключение во втором методе - createSoftwareWithOkAuthor () при save () автор. Почему это произошло?

org.hibernate.AssertionFailure: null id in models.Software entry (don't flush the Session after an exception occurs)
  at org.hibernate.event.def.DefaultFlushEntityEventListener.checkId(DefaultFlushEntityEventListener.java:82)
  at org.hibernate.event.def.DefaultFlushEntityEventListener.getValues(DefaultFlushEntityEventListener.java:190)
  at org.hibernate.event.def.DefaultFlushEntityEventListener.onFlushEntity(DefaultFlushEntityEventListener.java:147)
  at org.hibernate.event.def.AbstractFlushingEventListener.flushEntities(AbstractFlushingEventListener.java:240)
  at org.hibernate.event.def.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:99)
  at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:50)
  at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1206)