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Hibernate: persist an object containing a list with more objects

Joachim Rohde
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Joined: Nov 27, 2006
Posts: 423

After years of using iBatis I'm giving Hibernate a shot. My problem is that I'm not able to figure out how to persist an object which contains a list of other objects which are containing a back reference.

In my small little test-application I have an object called CD and an object called Track. Each CD should contain a list of tracks (one-to-many) and each Track should contain a back reference to the CD object to which it belongs to.

After trying out a few examples which I found on the internet and reading hours the documentation I'm desparated because I can't get it work.

Here's what I've got so far:

My trivial database schema contains only two tables:


My CD-object:


My track-object:


My test-application:



My hibernate.cfg.xml:


And the HibernateUtil-class which I copied from somewhere:


If I insert some data manually in the database the loading of the objects works perfectly. I can load a CD and the tracks are automatically loaded as well, just as expected.
If I try to save a CD-object with only one track included in the list it ALSO works as expected. Both objects are persisted and Hibernate sets the correct foreign-key (the field cdId in table track). But as soon as there are more objects in my list, I encounter a NonUniqueObjectException (Exception in thread "main" org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session).
I understood (I guess) that the problem is that my CD-object which I associate in the newCD()-method (via track.setCd(cd);) with a track is not unique in the hibernate-session.

Due to the fact that none of those objects exists in the database I can't load them from the database into the hibernate-session. I also tried to play around with evict() and merge() and also trying out a hell lot of annotation-combinations but none of them worked.

So how is this solved in general? Do I have to persist first the CD-object, load it again and then persist the Track-objects?

Sorry for this lengthy post but this really bugs me since days and I'm having the feeling is just small thing I don't see yet. And I don't know if the annotations are completly wrong or if the newCD()-method has to look differently. So, any hints for solving this problem are really really appreciated.

Joachim
Mark Spritzler
ranger
Sheriff

Joined: Feb 05, 2001
Posts: 17250
    
    6

Well, that is quite a bit there for something pretty simple.

So I would have this basic.





Also, in the latest versions of Hibernate, you no longer need the HibernateSingleton class anymore, just call sessionFactory.getCurrentSession()

That is all you need to make a call like
session.saveOrUpdate(cd);

to save the cd and all of its tracks.

Good Luck and above all, simplify, simplify, simplify

Mark


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Joachim Rohde
Ranch Hand

Joined: Nov 27, 2006
Posts: 423

How cool, it works. Thanks a lot. Even though I was not able yet to implement the lazy-loading, but I guess I have to read a bit more documentation for that.

But I still have a question though: if the HibernateSingleton class is not needed anymore and calling just sessionFactory.getCurrentSession() is considered best-practice now, where should I instantiate the sessionFactory object? Should it be instantiated in the constructor of each class, where I need access to the database? Or what's considered best-practice?
 
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