Bypass GeneratedValue in Hibernate (merge data not in db?)

My problem is the same as described in [1] or [2]. I need to manually set a by default auto-generated value (why? importing old data). As described in [1] using Hibernate's entity = em.merge(entity) will do the trick.

Unfortunately for me it does not. I neither get an error nor any other warning. The entity is just not going to appear in the database. I'm using Spring and Hibernate EntityManager 3.5.3-Final.

Any ideas?

Another implementation, way simpler.

This one works with both annotation-based or xml-based configuration: it rely on hibernate meta-data to get the id value for the object. Replace SequenceGenerator by IdentityGenerator (or any other generator) depending on your configuration. (The creation of a decorator instead of subclassing, passing the decorated ID generator as a parameter to this generator, is left as an exercise to the reader).

public class UseExistingOrGenerateIdGenerator extends SequenceGenerator {
    @Override
    public Serializable generate(SessionImplementor session, Object object)
                        throws HibernateException {
        Serializable id = session.getEntityPersister(null, object)
                      .getClassMetadata().getIdentifier(object, session);
        return id != null ? id : super.generate(session, object);
    }
}

Answer to the exercise (using a decorator pattern, as requested), not really tested:

public class UseExistingOrGenerateIdGenerator implements IdentifierGenerator, Configurable {

    private IdentifierGenerator defaultGenerator;

    @Override
    public void configure(Type type, Properties params, Dialect d) 
                        throws MappingException;
        // For example: take a class name and create an instance
        this.defaultGenerator = buildGeneratorFromParams(
                params.getProperty("default"));
    }

    @Override
    public Serializable generate(SessionImplementor session, Object object)
                        throws HibernateException {
        Serializable id = session.getEntityPersister(null, object)
                      .getClassMetadata().getIdentifier(object, session);
        return id != null ? id : defaultGenerator.generate(session, object);
    }
}

it works on my project with the following code:

@XmlAttribute
@Id
@Basic(optional = false)
@GeneratedValue(strategy=GenerationType.IDENTITY, generator="IdOrGenerated")
@GenericGenerator(name="IdOrGenerated",
                  strategy="....UseIdOrGenerate"
)
@Column(name = "ID", nullable = false)
private Integer id;

and

import org.hibernate.id.IdentityGenerator;
...
public class UseIdOrGenerate extends IdentityGenerator {
private static final Logger log = Logger.getLogger(UseIdOrGenerate.class.getName());

@Override
public Serializable generate(SessionImplementor session, Object obj) throws HibernateException {
    if (obj == null) throw new HibernateException(new NullPointerException()) ;

    if ((((EntityWithId) obj).getId()) == null) {
        Serializable id = super.generate(session, obj) ;
        return id;
    } else {
        return ((EntityWithId) obj).getId();

    }
}

where you basically define your own ID generator (based on the Identity strategy), and if the ID is not set, you delegate the generation to the default generator.

The main drawback is that it bounds you to Hibernate as JPA provider ... but it works perfectly with my MySQL project