Fluent NHibernate Uni-Directional one to one mapping

I cant get at One-to-One relationship working with Fluent NHibernate. I have User and UserDetails tables and they 'share' a primary key. How do I map them?

This is my latest/best attempt, this fails with NHibernate.Id.IdentifierGenerationException: attempted to assign id from null one-to-one property: User

User

 protected UserMap()
{
        Table("user");
        Id(x => x.Id)
        .Column("user_key")
       .GeneratedBy.GuidComb().UnsavedValue(Guid.Empty);

         References(x => x.UserDetail)
        .PropertyRef(x=>x.User)
        .Column("user_key")
        .Not.Insert().Not.Update().Cascade.All();

}

UserDetail

protected UserDetailMap()
{
     Table("user_detail");
     Id(x => x.Id).Column("user_key")
        .GeneratedBy.Foreign("User")
        .UnsavedValue(Guid.Empty);

     References(x => x.User开发者_如何学运维)
        .Column("user_key")
        .Not.Insert().Not.Update().Unique().Cascade.None();

}

Try this

protected UserDetailMap()
{
    Table("user_detail");
    Id(x => x.Id)
       .Column("user_key")
       .GeneratedBy.Foreign("User")
       .UnsavedValue(Guid.Empty);

    HasOne(x => x.User)
       .Constrained();    
}