Get previous version of entity in Hibernate Envers

Question

I have an entity loaded by Hibernate (via EntityManager):

User u = em.load(User.class, id)

This class is audited by Hibernate Envers. How can I load the previous version of a User entity?

Answer 1

Here's another version that finds the previous revision relative to a "current" revision number, so it can be used even if the entity you're looking at isn't the latest revision. It also handles the case where there isn't a prior revision. (em is assumed to be a previously-populated EntityManager)

public static User getPreviousVersion(User user, int current_rev) {
    AuditReader reader = AuditReaderFactory.get(em);

    Number prior_revision = (Number) reader.createQuery()
    .forRevisionsOfEntity(User.class, false, true)
    .addProjection(AuditEntity.revisionNumber().max())
    .add(AuditEntity.id().eq(user.getId()))
    .add(AuditEntity.revisionNumber().lt(current_rev))
    .getSingleResult();

    if (prior_revision != null)
        return (User) reader.find(User.class, user.getId(), prior_revision);
    else
        return null
}

This can be generalized to:

public static T getPreviousVersion(T entity, int current_rev) {
    AuditReader reader = AuditReaderFactory.get(JPA.em());

    Number prior_revision = (Number) reader.createQuery()
    .forRevisionsOfEntity(entity.getClass(), false, true)
    .addProjection(AuditEntity.revisionNumber().max())
    .add(AuditEntity.id().eq(((Model) entity).id))
    .add(AuditEntity.revisionNumber().lt(current_rev))
    .getSingleResult();

    if (prior_revision != null)
        return (T) reader.find(entity.getClass(), ((Model) entity).id, prior_revision);
    else
        return null
}

The only tricky bit with this generalization is getting the entity's id. Because I'm using the Play! framework, I can exploit the fact that all entities are Models and use ((Model) entity).id to get the id, but you'll have to adjust this to suit your environment.