I have collection mapped with cascade = "all-delete-orphan".

I remove some object from this collection.

Then I create another one with the same primary key as the one removed and add this object to collection.

When I commit this transaction NonUniqueObjectException is thrown.

With this cascade option Hibernate keeps tracking changes to collection and to instances of objects loaded in the same transaction until transaction gets commited and session closed. Your previously loaded instance after deletion from collection was marked as deleted and will be deleted from DB if you save parent and commit transaction. Your new object will be as a new for Hibernate. Hibernate not smart enough to determine that actually you are trying to execute update (and/or delete and insert) Reuse your instance if your are not going to delete it. Or, create a new one with different ID.

I suppose this is some bug that can be fixed.

In hibernate all database changes are executed in this order: insert, update, delete.

This problem araises because deletes are after inserts.

When I call flush before adding new object to collection transaction complets sucessfull.

I know that Hibernate detects this kind of situation when I use explicite delete and save - then flush is automaticaly invoked in save method before inserting new object when there is another one with the same id that is marked as deleted.

Unfortunatly for collection this doesn't work. Isn't it possible to fix this?

No.