NHibernate relationships with a shared composite key

I have two tables with composite keys and a linking table between them, like so:

• A

  • aid (PK)
  • sharedkey (PK)

• B

  • bid (PK)
  • sharedkey (PK)

• Linking

  • aid
  • bid
  • sharedkey
  • sortorder

So the awkward thing there is the shared key, which is part of both FKs. Here's the first thing I tried (I'm using Fluent, but I would have this same problem with XML mappings):

mapping.HasManyToMany(x => x.Bees)
    .Table("Linking")
    .ParentKeyColumns.Add("aid", "sharedkey")
    .ChildKeyColumns.Add("bid", "sharedkey")
    .OrderBy("sortorder");

Which gives me an mapping exception: "Repeated column in mapping for collection", because I've put in sharedkey twice. That makes sense, because I haven't told in NH that I actually want sharedkey to be the same in each direction.

The Java version of Hibernate has a formula feature that solves this exactly, but NH, AFAIK, does not. So how开发者_开发知识库 can I map this?

Some caveats:

1. It's not totally necessary for this to be in Fluent. XML is fine too
2. This is all read-only, so if the solution breaks inserts in some funny way, that's fine
3. I can't change the DB at all

a hackish solution could be

public AMap()
{
    CompositeId()
        .KeyProperty(x => x.Id, "aid")
        .KeyProperty(x => x.Name, "sharedkey");

    HasManyToMany(x => x.Bees)
        .Table("Linking")
        .ParentKeyColumns.Add("aid", "sharedkey")
        .ChildKeyColumn("bid").ChildPropertyRef("Id")
        .OrderBy("sortorder")
        ;
}

public BMap()
{
    CompositeId()
        .KeyProperty(x => x.Id, "bid")
        .KeyProperty(x => x.Name, "sharedkey");

    Map(x => x.Id, "bid")
        .Not.Insert();
}

to get all B's this will result in (pseudo sql):

SELECT l.bid FROM a LEFT JOIN linking l ON a.aid = l.aid AND a.sharedkey = l.sharedkey WHERE a.aid = ?

this will get all B with the given aid and the same sharedkey.