CompositeId()
        .KeyProperty(x => x.Id)
        .KeyProperty(x => x.Type);

For the other entity, I've tried the following:

References(x => x.EntityWith2ColsPK);

And failed with:

    Foreign key (Fk_MyEntity_EntityWith2ColsPK:MyEntities
[Fk_EntityWith2ColsPK])) must have same number of columns as the
referenced primary key (EntityWith2ColsPKs [Id, Type])

How can I reference EntityWith2ColsPK from another entity?

I've tried this:

    HasMany<EntityWith2ColsPK>(x =>
x.EntityWith2ColsPK).KeyColumns.Add("Id", "Type").Cascade.All();

Which failed with:

    Custom type does not implement UserCollectionType: Category

But anyway I don't want a 1 to many relation, I want a 1 to 1
relation. Still, I can't make either of them work.

Also, I've tried:

    HasOne<EntityWith2ColsPK>(x => x.EntityWith2ColsPK).PropertyRef(x
=> x.Id).PropertyRef(x => x.Type);

Which fails with:

    NHibernate.MappingException : property not found: Language on
entity AFR.Domain.Entities.Category

What can I do for this to really work?