I'm at the point where I have a webservice up and running that allows me to save a single object to the database, via an EntityManager. After that, I'm having some problems.

1) em.contains() does not seem to work. I have a web service "insertOrUpdate" which does the following:



When I run it the first time, em.contains() returns false and it does a persist. When I run it the second time, using the same object, em.contains() still returns false and the persist fails because the entity is already in the DB.

2) em.merge() does not delete items in a collection (inside the entity) that are no longer there, it simply adds any additional ones it finds. Is there an easy way to do this or do I have to remove an entity and persist it again?

3) Why does em.remove() require an entity as an argument. Isn't it enough to just have the entity's primary key and be done with it? Or do I have to do something like get the entity via em.find() then use the entity it returns and do a em.remove() on that entity.

Try to use a HQL query instead, because this will try to lookup the entity in the database instead of only in the cache of entitymanager.


Well this is exactly the problem i'm having. I suppose you have set cascade to DELETE. Deleting items in the collection does not invoke any cascade delete. For this you will have to delete the owning entity (server in your case), like you say. But there must be an easier way. Currently I'm using a for loop to delete every item manually. Have you come up with a solution yet?



Well. You can of course use a delete HQL statement, like Delete FROM server where id =:id

I'm actually working on another project, as this one has been put on hold. :(

I was able to deal with deleting collections by making the objects "Embeddable" and the collection as a "CollectionOfElements". That way, not only did I not have to give the objects a unique ID, but they were automatically deleted when the parent object was removed, and they were automatically deleted/inserted when the parent object was merged.

The only drawback was that you could only do this down 1 level. You can't have an embeddable object that contains other embeddable objects...only simple types. At least, I couldn't get it to work when I tried it.

Thanks for the HQL tips. I never delved into that aspect of Hibernate but, next time I actually get a project that requires persistance, I'll be sure to check it out.