NHibernate configuration for uni-directional one-to-many relation

Question

I'm trying to set up a relationship as follows. Each Master item has one or more Detail items:

public class Detail {
    public virtual Guid DetailId { get; set; }
    public virtual string Name { get; set; }
}
public class Master {
    public virtual Guid MasterId { get; set; }
    public virtual string Name { get; set; }
    public virtual IList<Detail> Details { get; set; }
}

And Mappings:

public class MasterMap : ClassMap<Master> 
{
    public MasterMap() 
    {
        Id(x => x.MasterId);
        Map(x => x.Name);
        HasMany(x => x.Details).Not.KeyNullable.Cascade.All();
    }
}
public class DetailMap : ClassMap<Detail> 
{
    public DetailMap() 
    {
        Id(x => x.Id);
        Map(x => x.Name);
    }
}

The Master database table is:

masterId   uniqueidentifier NOT NULL
name       nvarchar(max) NULL

and Detail is:

DetailId   uniqueidentifier NOT NULL
name       nvarchar(max) NULL
MasterId   uniqueidentifier NULL
foreign key (masterId) references [Master]

I don't really care to have a link from Detail back to Master -- in otherwords, Detail objects on their own are just not interesting to my domain layer. They will always be accessed via their Master object.

Using code like this:

Master mast = new Master 
{
    MasterId = new Guid(),
    Name = "test",
    Details = new List<Detail> 
    {
        new Detail { .DetailId = new Guid(), .Name = "Test1" },
        new Detail { .DetailId = new Guid(), .Name = "Test1" }
    }
};

using (transaction == Session.BeginTransaction) 
{
    Session.Save(mast);
    transaction.Commit();
}

This works great, except for a crazy limitation outlined in this post: NHibernate does an INSERT and puts Detail.MasterId as NULL first, then does an UPDATE to set it to the real MasterId.

Really, I don't want Detail entries with NULL MasterIds, so if I set the MasterId field to NOT NULL, the INSERT to Detail will fail, because as I said NHibernate is trying to put in MasterId = NULL.

I guess my question boils down to this:

How can I get the above code sample to work with my existing domain model (eg, without adding a Detail.Master property), and the Detail.MasterId field in the database set to NOT NULL?

Is there a way to get Nhibernate to just put the correct MasterId in the initial INSERT, rather than running an UPDATE afterwards? Is there rationale somewhere for this design decision? -- I'm struggling to see why it would be done this way.

Answer 1

You can't. To quote the link from my answer on the other question you linked to:

Very Important Note: If the <key> column of a <one-to-many> association is declared NOT NULL, NHibernate may cause constraint violations when it creates or updates the association. To prevent this problem, you must use a bidirectional association with the many valued end (the set or bag) marked as inverse="true". See the discussion of bidirectional associations later in this chapter.

Edit: as Hazzik has rightly pointed out, this has changed in NHibernate 3 and above. The docs sadly haven't been updated, so here's Hazzik:

[If you] set inverse="false" and not-null on <key>, NH3 and above will perform only two inserts insead of insert-insert-update.

Answer 2

NH3 and above allow to correct save entities in case of uni-directional one-to-many mapping without annoying save null-save-update cycle, if you set both not-null="true" on <key> and inverse="false" on <one-to-many>

FluentNHibernate code snippet for that:

public class MasterMap : ClassMap<Master> 
{
    public MasterMap() 
    {
        Id(x => x.MasterId);
        Map(x => x.Name);
        HasMany(x => x.Details)
            .Not.Inverse()     //these options are very
            .Not.KeyNullable() //important and work only if set together 
            .Not.KeyUpdate()   //to prevent double update
            .Cascade.All();
    }
}

Answer 3

The reason NHibernate does it this way is because:
When it saves the detail it only knows about the stuff the detail knows about. So any master references which happen in the background are ignored. Only when the master is saved it sees the relation and updates the elements of the collection with the id of the master.
Which is from an object oriented point of view logical. However from a saving point-of-view is slightly less logical. I suppose you can always file a bug report, or look if it might have been filed already and ask them to change it. But I suppose they have their specific (design/domain) reasons.