Adding object to collection outside of unit of work?

Question

Hi,
Tried to find the answer in this forum but difficult to know what keywords to use...anyway
What I am trying to do is so simple I can only believe I am missing the point somewhat ;-)
I have an object Licence that has a Set of LicenceHolders. I have a licence already saved in the db and the application now has cause to add a licenceHolder. I create the licenceHolder as a new object then get the licence out of the db, add the licenceHolder to the Set, then I make a call to my data persistance business class to make the save to the database. I.e. I have a separate layer to do the persistance from my business logic. The latter being the specific method to say 'add a licence holder to the licence', the former being 'save the licenceHolder, and update the link table between the licence and licenceHolder' (m - m relationship).
The problem I have is that in my business method licence.addLicencee(licenceHolder) doesn't save my licenceHolder and link table if I don't include the bold line below. This does work but see below for more comments
code snippet for business method....
// lm is the persistance method that gets licence from db
PremisesLicence licence = (PremisesLicence) lm.getLicence(premisesLicenceId);
// licenceePerson - new object not yet persisted
licence.addLicenceHolder(licenceePerson);     
//lhm is another persistance method to save licenceePerson and update licence_licenceHolder link table
lhm.addLicenceHolder(licenceePerson, licence);
code for lhm...
public void addLicenceHolder(ILicenceHolder licenceHolder, Licence licence) throws DataAccessException {
UnitOfWork uow = aSession.acquireUnitOfWork();
try {
     Licence licenceClone = (Licence) uow.readObject(licence);
licenceClone.addLicenceHolder(licenceHolder);
     uow.registerNewObject(licenceHolder);
     uow.commit();
} catch (Exception e) {
     e.printStackTrace();
     throw new DataAccessException(e);
I don't believe I should have to do the bold line as it is business logic in the persistance logic. I have already (in business method) said 'this licence has this licenceHolder' why should I have to repeat it in the persistance layer too. This can only lead to bugs in my software - I need all the help I can not to introduce any more ;-)
Comments please?
Thanks
Conrad

Answer 1

We've been working with TopLink for a while now, and had some issues with what you are doing. The following way is how we interact with TopLink to enable modifications outside the UoW:
- When returning objects from you DAO layer always return a copy of the retrieved objects (try Session.copyObject()), not the object returned by TopLink.
- Do modifications to the copied objects
- Pass the modified object graph to the DAO layer and use uow.mergeCloneWithReferences() to merge in differences
- Save to database with TopLink
From our experience, the reason you have to copy things is because what TopLink returns is the cached objects. So because you then will modify the cached objects, TopLink will not discover changes (comparing an object to itself will not give any differences). At least that was the only explanation we could find to our strange behaviour.
This is not a very efficient way to do things, and I think that if you are able to have a UoW open during this whole process it would be much more efficient. For an easy way to do this with web applications check out the TopLink integration with Springframework.
Please let me know if anyone has any feedback to this way of doing it. I know it's not ideal, but it is the best solution we found for detaching the objects.