When I run into trouble like that, the first thing I twiddle with is the cascade option.
Ken
2010-01-04 23:42:20
A:
Hi Ken, thanks. I have tried all the possible options from that article. I think maybe my solution is incorrect.
Daoming Yang
2010-01-04 23:47:22
A:
As far as I know, the mapping is correct (I'm using mapping files that look exactly the same). The problem is the cascade attribute: the "all" option forces NHibernate to propagate each action on an entity to the instances of the collection. In your case, when you delete a news item all related tags are deleted too.
You probably should use "none" (in that case you'll eventually end up with some unused tags in the database) or "delete-orphans" (on the news item side - use "none" on the tag side).
Lck
2010-01-04 23:50:45
Hi Lck, I have tried your suggestion and got the following errors: "object references an unsaved transient instance - save the transient instance before flushing." Any thoughts?
Daoming Yang
2010-01-05 00:18:21
Yes, forgot about that. Since NHibernate isn't cascading updates to the collection, the Tag instances will not be updated. If you add a new tag to the collection, that instance will not be persisted and NHibernate will fail trying to create a link to it. The solution in this case is to manually SaveOrUpdate() each tag instance when you persist a news item.Kind of clunky, but it's a trade-off: other cascade settings persist entities automatically but can cause other issues.
Lck
2010-01-05 13:21:29
A:
Use the cascade option "save-update".
The option "all" will cascade deletes, which you do not want in this case. But you the option "none" will require that the Tag entity is already persisted which I guess might not always be the case.
So by setting the cascade to "save-update" new Tags till be inserted in the Tags table and in the link table News_Tags, but when you remove a tag from a News entity it will only be removed from the link table not the Tags table.
Torkel
2010-01-05 08:38:43
Hi Torkel, I have tried your solution which to set the cascade="save-update" for news mapping and tag maping. It has two problem: 1. when I do news update including the tag updating. It did updates, but it did not remove the old tag from the link table; 2. When I try to delete the news it gave me the error: "deleted object would be re-saved by cascade (remove deleted object from associations)". I'm very confusing about this. Any other thoughts?
Daoming Yang
2010-01-05 11:49:55