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

Question

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?

Answer 1

You need a running transaction.

In case your transaction are manually-managed:

entityManager.getTransaction().begin();

(of course don't forget to commit)

If you are using declarative transactions, use the appropriate declaration (via annotations, most likely)

Also, set the hibernate logging level to debug (log4j.logger.org.hibernate=debug) in your log4j.properties in order to trace what is happening in more details.

Answer 2

According to the Selectively disable generation of a new ID thread on the Hibernate forums, merge() might not be the solution (at least not alone) and you might have to use a custom generator (that's the second link you posted).

I didn't test this myself so I can't confirm but I recommend reading the thread of the Hibernate's forums.

Answer 3

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

Answer 4

For anyone else looking to do this, above does work nicely. Just a recommendation to getting the identifier from the object rather than having inheritance for each Entity class (Just for the Id), you could do something like:

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 object)
            throws HibernateException {
        if (object == null)
            throw new HibernateException(new NullPointerException());

        for (Field field : object.getClass().getDeclaredFields()) {
            if (field.isAnnotationPresent(Id.class)
                    && field.isAnnotationPresent(GeneratedValue.class)) {
                boolean isAccessible = field.isAccessible();
                try {
                    field.setAccessible(true);
                    Object obj = field.get(object);
                    field.setAccessible(isAccessible);
                    if (obj != null) {
                        if (Integer.class.isAssignableFrom(obj.getClass())) {
                            if (((Integer) obj) > 0) {
                                return (Serializable) obj;
                            }
                        }
                    }
                } catch (IllegalArgumentException e) {
                    e.printStackTrace();
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
        }

        return super.generate(session, object);
    }
}