Comments on: Note on sets in hibernate/nHibernate

By: temojin

temojin — Tue, 12 Aug 2008 23:30:56 +0000

It's possible to set the index to some arbitrary starting value. The problem i have is when i save the collection, the first row's index value is set to 0. So when i retrieve the collection (via the owning parent object) i get an array out of bounds exception. Has anyone else notice this? The mapping would look like this:

Notice in my case i made the inverse="false" to let this side manage the persistence of the object (on the many-to-one side i put update="false" insert="false"

By: jlockwood

jlockwood — Fri, 06 Jun 2008 13:55:33 +0000

@Ben

Right, I realize that. As far as the project that I’m working on, the DBA doesn’t want it changed (see my response above). It’s a little wonky, but I have it working with the way our code is structured. I’ll change the example now to reduce confusion between this project and the “real world”. Thanks for the comment!

By: Ben Scheirman

Ben Scheirman — Fri, 06 Jun 2008 12:28:18 +0000

By definition Sets are not ordered. What you want is a List, as James Kovacs mentioned above.

By: jlockwood

jlockwood — Fri, 06 Jun 2008 12:27:23 +0000

@Brian
I’ve got good test coverage and all is passing, but I do see what you are saying and will look into it. I do think that List<> should be used and not Set (usually use list by default with NHiberante). I’ll go ahead and change the example to reflect this. I believe I would still need the ‘order by’ in the mapping, right?

@James
I hear you. As stated above I’ll most likely switch to list. I really don’t want to implement Comparable right now. I think the use of set in the first place was wrong, but I see that it resulted from the process used to generate the ‘Thing’ class.

@Sean
I wasn’t aware of that, thanks. Note that the sequence number in the example above is not a database sequence (like in Oracle).

@All
The app that I’m working on is a little funky and one could argue that the process used in working on the data end is unecessary. I was trying to minimize needed changes to the hbms because of our process (which I’m hoping to change).

The process is as follows:
0. ValueObjects are created for use in the service layer.
1. Model the DB using Erwin to loosely support the VOs.
2. Generate the DDL’s from the model in Erwin.
3. Generate the hbms from the DDLs.
4. Generate the DataObjects from the hbms.
5. Create conversion classes to map between the VOs and DOs.

I’m new to the project and this is my first go at Hibernate. I have quite a bit of NHibernate experience under my belt. The approach we are using is reverse of the process I would normally use (i.e. creating DomainEntities first, then hbms, then schema generation [DDLs]).

There’s a huge risk with this approach since every time new hbms are generated to support a VO we need to massage the hbm file. We try to only generate the hbms/DOs that are needed due to schema changes. This whole thing is pretty brittle and there’s a huge risk of breaking things with each schema change.

By: Sean Carpenter

Sean Carpenter — Fri, 06 Jun 2008 11:19:39 +0000

Hibernate supports an arbitrary base for by specifying the “base” attribute (it doesn’t have to be zero-based). NHibernate also supports this in the upcoming 2.0 release (currently in alpha).

By: James Kovacs

James Kovacs — Fri, 06 Jun 2008 03:07:44 +0000

I'm coming from the NHibernate world, but shouldn't you be mapping this with a list and using an index element? Order-by is SQL and performs the sort in the database by augmenting the SQL query. This means that items added to the collection won't be in the proper sort order until persisted and reloaded. Using a , items are always in the proper order.

The one limitation is that has to be zero-based. If you don't have a zero-based index, then I would use the sort attribute on to specify an IComparer to ensure proper ordering. A quick read over the Hibernate docs seems to confirm that Hibernate has similar facilities though attribute/element names are slightly different in cases. See Hibernate docs section 6.2.3 and .

By: Brian Yarger

Brian Yarger — Fri, 06 Jun 2008 02:50:14 +0000

In order for this to work, you’ll also need to make sure that the collection class holding “myCollectionOfThings” preserves order. A HashSet will not, but a LinkedHashSet will, or of course a List would.