Hibernate, unidirectional ManyToOne and the desire for a “On Delete Cascade” Feature

Question

I have a problem that's equal to the one presented here: how to define an inverse cascade delete on a many-to-one mapping in hibernate

After searching for a while I can't find a decent/clean solution for this. I can't have the parent entity have an @OneToMany to the child, because they are at different modules. I wanted to try the EntityListener that would delete the children before the parent is, but I can't because, again, they are at different modules.

Does anyone know a clean solution to this? I am thinking about to use AspectJ to listen to the call of the delete method from the ParentDao, but this is not a clean solution and I will have to implement one for each entity that have this kind of relation to the Parent class.

This kind of cascade seems to be a basic feature and I am kind dissapointed in seeing that hibernate does not support it :/

Answer 1

Based on JB Nizet answer I changed my DAO to have a DeleteOperationListener (My base DAO implementation is based on "Don't Repeat the DAO"[1].) In this way I have a generic solution in case I find myself in the same situation again. The structure looks like this:

public interface GenericDao<T, PK extends Serializable> {
    // CRUD methods

    // delete operation listeners.
    void addDeleteListener(DeleteOperationListener<T, PK> deleteOperationListener);

    public interface DeleteOperationListener<T> {
        void preDelete(T entity);
        void posDelete(T entity);
    }
}

And my abstract hibernate implementation I can notify the observers about the delete.

@Override
public void delete(T entityToDelete) {
    notifyPreDelete(entityToDelete);
    this.getHibernateTemplate().delete(entityToDelete);
    notifyPosDelete(entityToDelete);
}

And now I have a different class that handles the deletion of the children without needing to change the DAOs:

@Service
public class ParentModificationListener
    implements GenericDao.DeleteOperationListener<Parent> {

    private ChildDao childDao;

    @Autowired
    public ParentModificationListener(ChildDao childDao, ParentDao parentDao) {
        this.childDao = childDao;
        parentDao.addDeleteListener(this);
    }

    @Override
    public void preDelete(Parent parent) {
        this.childDao.deleteChildrenFromParent(parent);
    }

    @Override
    public void posDelete(Parent parent) {
        // DO NOTHING
    }
}

[1] http://www.ibm.com/developerworks/java/library/j-genericdao.html

Answer 2

The answer of the question you linked to is correct. Hibernate can only delete the children when deleting the parent if the parent knows about its children.

The only solution is to have the ParentDAO's delete method to search for all the children of the parent, delete them, and then delete the parent itself.

If your concern is that the ParentDAO shouldn't know about the children, you could make it decoupled, and have the ParentDAO have a list of registered ParentDeletionListeners, which would be invoked before deleting the parent itself. The ParentDAO know only about this ParentDeletionListener interface, and allows registering several listeners. When starting the application, register a listener for every kind of child, and have the listener delete the children:

public interface ParentDeletionListener {
    void parentWillBeDeleted(Parent parent);
}

public class SomeChildParentDeletionListener implements ParentDeletionListener {
    // ...
    public void parentWillBeDeleted(Parent parent) {
        // search for every SomeChild linked to the given parent
        // and delete them
    }
}

public class ParentDAO {
    private List<ParentDeletionListener> listeners = new CopyOnWriteArrayList();

    public void addParentDeletionListener(ParentDeletionListener listener) {
        this.listeners.add(listener);
    }

    public void deleteParent(Parent p) {
        for (ParentDeletionListener listener : listeners) {
            listener.parentWillBeDeleted(parent);
        }
        session.delete(parent);
    }
}