javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: detached entity passed to persist: test.PersistMergeTest$User
   at org.hibernate.ejb.AbstractEntityManagerImpl.throwPersistenceException(AbstractEntityManagerImpl.java:562)
   at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:193)
   at test.PersistMergeTest.persist(PersistMergeTest.java:59)
   at test.PersistMergeTest.testPersistObjectWithDetachedObjectReference(PersistMergeTest.java:158)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
   at java.lang.reflect.Method.invoke(Unknown Source)
   at junit.framework.TestCase.runTest(TestCase.java:154)
   at junit.framework.TestCase.runBare(TestCase.java:127)
   at junit.framework.TestResult$1.protect(TestResult.java:106)
   at junit.framework.TestResult.runProtected(TestResult.java:124)
   at junit.framework.TestResult.run(TestResult.java:109)
   at junit.framework.TestCase.run(TestCase.java:118)
   at junit.framework.TestSuite.runTest(TestSuite.java:208)
   at junit.framework.TestSuite.run(TestSuite.java:203)
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:478)
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:344)
   at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)
Caused by: org.hibernate.PersistentObjectException: detached entity passed to persist: test.PersistMergeTest$User
   at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:79)
   at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:608)
   at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:600)
   at org.hibernate.engine.CascadingAction$8.cascade(CascadingAction.java:202)
   at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:213)
   at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:157)
   at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:108)
   at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:290)
   at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:185)
   at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:160)
   at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:108)
   at org.hibernate.engine.Cascade.cascade(Cascade.java:248)
   at org.hibernate.event.def.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:437)
   at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:326)
   at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:180)
   at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:121)
   at org.hibernate.event.def.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:131)
   at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:87)
   at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:38)
   at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:617)
   at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:591)
   at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:595)
   at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:187)
   ... 17 more

package test;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Persistence;

import junit.framework.TestCase;

import org.hibernate.annotations.GenericGenerator;

public class PersistMergeTest extends TestCase
{
   private EntityManagerFactory emf = Persistence.createEntityManagerFactory("manager1");;

   @Entity
   static class User
   {
      @Id
      @GeneratedValue(generator = "system-uuid")
      @GenericGenerator(name = "system-uuid", strategy = "uuid")
      String id;

      @ManyToOne(optional = true, fetch = FetchType.EAGER)
      Group group;
   }

   @Entity
   static class Group
   {
      @Id
      @GeneratedValue(generator = "system-uuid")
      @GenericGenerator(name = "system-uuid", strategy = "uuid")
      String id;

      @OneToMany(mappedBy = "group", fetch = FetchType.EAGER, cascade = {CascadeType.PERSIST,
            CascadeType.MERGE, CascadeType.REFRESH})
      Set<User> users = new HashSet<User>();
   }

   private void persist(Object e) throws RuntimeException
   {
      EntityManager em = emf.createEntityManager();
      EntityTransaction tx = null;
      try
      {
         tx = em.getTransaction();
         tx.begin();
         em.persist(e);
         tx.commit();
      }
      catch (RuntimeException ex)
      {
         if (tx != null && tx.isActive()) tx.rollback();
         throw ex;
      }
      finally
      {
         em.close();
      }
   }

   private void merge(Object e) throws RuntimeException
   {
      EntityManager em = emf.createEntityManager();
      EntityTransaction tx = null;
      try
      {
         tx = em.getTransaction();
         tx.begin();
         em.merge(e);
         tx.commit();
      }
      catch (RuntimeException ex)
      {
         if (tx != null && tx.isActive()) tx.rollback();
         throw ex;
      }
      finally
      {
         em.close();
      }
   }

   /* FAILS */
   public void testPersistObjectWithDetachedObjectReference()
   {
      // instantiate and persist a user object
      User aUser = new User();
      persist(aUser);
      assertNotNull(aUser.id);

      // instantiate a group object
      Group aGroup = new Group();

      // assign the detached user object to the new group object
      aGroup.users.add(aUser);
      aUser.group = aGroup;

      // **************************************
      // the following statement will throw javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: detached entity passed to persist: test.UserEntity
      // **************************************
      persist(aGroup);
   }

   /* WORKS */
   public void testMergeObjectWithDetachedObjectReference()
   {
      // instantiate and persist a user object
      User aUser = new User();
      persist(aUser);
      assertNotNull(aUser.id);

      // instantiate and persist a group object
      Group aGroup = new Group();
      persist(aGroup);
      assertNotNull(aGroup.id);

      // assign the user to the group and merge the group object
      aGroup.users.add(aUser);
      aUser.group = aGroup;
      merge(aGroup);
   }
}