Auto generated domain class id with exceptions

Question

Scenario: Domain class is created and inserted using the default Postgres sequence generator (with a custom dialect to create one sequence per table but I think this is irrelevant). Now, in some cases I'd like to choose my own ID for insertion.

I imagine two basic ways:

• preferred: simply set the id in the domain prior saving. If it is not set, use a generated id. I naively tried this, but it didn't work, i.e. the instance is inserted with the auto generated id, even if another id is already set in the domain instance.
• using a "generator: assigned" mapping and determine a default id (let the DB decide). How to do this best?

Answer 1

OK, so finally I came up with this (basically a combination from diverse suggestions made by others at different points in time)

package org.hibernate.id;

import java.io.Serializable;
import java.util.Properties;

import org.hibernate.HibernateException;
import org.hibernate.dialect.Dialect;
import org.hibernate.engine.SessionImplementor;
import org.hibernate.type.Type;

public class UseExistingOrGenerateTabelNameSequenceIdGenerator extends org.hibernate.id.enhanced.SequenceStyleGenerator {

/**
 * {@inheritDoc} If the instance to insert does contain an ID
 * we use that id instead of an auto generated one.
 */
@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);
}

/**
 * {@inheritDoc} If the parameters do not contain a
 * {@link SequenceStyleGenerator#SEQUENCE_PARAM} name, we assign one based on the
 * table name.
 */
@Override
public void configure(final Type type, final Properties params, final Dialect dialect) {
    if (params.getProperty(SEQUENCE_PARAM) == null  || params.getProperty(SEQUENCE_PARAM).length() == 0) {
        String tableName = params.getProperty(PersistentIdentifierGenerator.TABLE);
        if (tableName != null) {
            params.setProperty(SEQUENCE_PARAM, "seq_" + tableName);
        }
    }
    super.configure(type, params, dialect);
}

}

To use it specifically on a domain class, you would include something like

static mapping = {
    id generator:'org.hibernate.id.UseExistingOrGenerateTabelNameSequenceIdGenerator', params:[optimizer:'pooled', increment_size:10]
}

into your domain class definition, which also allows to specify an id pool and its size. Seems to run great so far :)