public class A
{
   private Long id;
   private Integer version;
   private B b1;
   private B b2;
   
   public B getB1() { return b1; }
   public void setB1(B b1) { this.b1 = b1;   }

   public B getB2() { return b2;}
   public void setB2(B b2) { this.b2 = b2;   }

   public Long getId() { return id; }
   public void setId(Long id) { this.id = id; }

   public Integer getVersion() { return version; }

   public void setVersion(Integer version) { this.version = version; }
}

public class B {
   private Long id;
   private Integer version;
   private String description;
   
   public String getDescription() { return description; }
   public void setDescription(String description) { this.description = description; }

   public Long getId() { return id; }
   public void setId(Long id) { this.id = id; }

   public Integer getVersion() { return version; }

   public void setVersion(Integer version) { this.version = version; }
   
   public boolean equals(Object obj) { ... }
   public int hashCode() { ... }
}

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
   "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="fr.test.domain">
   <class name="A" table="A">
      <id name="id" column="A_ID">
         <generator class="increment" />
      </id>
      <version name="version" column="VERSION">
      </version>
      <many-to-one name="b1" cascade="all" class="fr.test.domain.B" />
      <many-to-one name="b2" cascade="all" class="fr.test.domain.B" />
   </class>
</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
   "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="fr.test.domain">
  <class name="B" table="B">
      <id name="id" column="B_ID">
         <generator class="increment" />
      </id>
      <version name="version" column="VERSION">
      </version>
      <property name="description" column="DESCRIPTION"/>
  </class>
</hibernate-mapping>

public class Test extends TestCase
{
   private A a;
   
        protected void setUp() throws Exception
   {
        //init
        a = init();
       
        //load B1
        B newB1 = loadB(a.getB2().getId());
        newB1.setDescription("b1");
        a.setB1(newB1);
   }
   
   public void testSaveOrUpdate()
   {
      assertNotNull(a);
      
   //   throws [b]NonUniqueObjectException[/b]
   //
      saveA(a);
   }
   
   public void testMerge()
   {
        assertNotNull(a);
      
        // Works but...
        a = mergeA(a);

        // .... [b]b1 modification is lost[/b]
        assertEquals("b1", a.getB1().getDescription());
   }
   
   //-------------------------------------------------------------------------
   // Internal methods
   //-------------------------------------------------------------------------
   protected A init() {
        Session session = HibernateUtil.getSessionFactory().openSession();
        Transaction tx = session.beginTransaction();

        A a = new A();
        B b = new B();
        b.setDescription("b");
        a.setB1(b);
        a.setB2(b);
        session.save(a);
      
        tx.commit();
        session.close();
        return a;
   }

   protected void saveA(A a)
   {
        Session session = HibernateUtil.getSessionFactory().openSession();
        Transaction tx = session.beginTransaction();
       
        session.saveOrUpdate(a);
       
        tx.commit();
        session.close();
   }
   
   protected A mergeA(A a)
   {
        Session session = HibernateUtil.getSessionFactory().openSession();
        Transaction tx = session.beginTransaction();
      
        A result = (A) session.merge(a);
       
        tx.commit();
        session.close();
       
        return result;
   }
   
   protected B loadB(Long id)
   {
        Session session = HibernateUtil.getSessionFactory().openSession();
        Transaction tx = session.beginTransaction();

        B b = (B)session.get(B.class, id);
   
        tx.commit();
        session.close();
        return b;
   }
}