Associating an Existing Source to an Existing Target Object

This section explains how to associate an existing source object with an existing target object with one-to-many and one-to-one relationships.

As shown in Example 101-9, associating existing objects with each other in a unit of work is as simple as associating objects in Java. Just remember to only work with working copies of the objects.

Example 101-9 Associating an Existing Source to Existing Target Object

// Associate all VetVisits in the database to a Pet from the database
UnitOfWork uow = session.acquireUnitOfWork();
    Pet existingPetClone = (Pet)uow.readObject(Pet.class);
    Vector allVetVisitClones;
    allVetVisitClones = (Vector)uow.readAllObjects(VetVisit.class);
    Enumeration enum = allVetVisitClones.elements();
    while(enum.hasMoreElements()) {
        VetVisit vetVisitClone =(VetVisit)enum.nextElement();
        existingPetClone.getVetVisits().addElement(vetVisitClone);
        vetVisitClone.setPet(existingPetClone);
    };
uow.commit();

The most common error when associating existing objects is failing to work with the working copies. If you accidentally associate a cache version of an object with a working copy you will get an error at commit time indicating that you associated an object from a parent session (the cache version) with a clone from this unit of work.

Example 101-10 shows another example of associating an existing source to an existing target object.

Example 101-10 Associating Existing Objects

// Get an employee read from the parent session of the unit of work
Employee employee = (Employee)session.readObject(Employee.class)

// Acquire a unit of work
UnitOfWork uow = session.acquireUnitOfWork();
Project project = (Project) uow.readObject(Project.class);

/* When associating an existing object (read from the session) with a clone, we must make sure we register the existing object and assign its clone into a unit of work */

/* INCORRECT: Cannot associate an existing object with a unit of work clone. A QueryException will be thrown */
//project.setTeamLeader(employee);

/* CORRECT: Instead register the existing object then associate the clone */
Employee employeeClone = (Employee)uow.registerObject(employee);
project.setTeamLeader(employeeClone);
uow.commit();