Hi, I'm trying to setup a one-to-one mapping from my Users to the UserDetails table. Say I have the following tables in my database:

Users:

- UserID (PK, Identity)
- UserName
- Password

UsersDetails:

- UserID (PK, FK)
- FirstName
- LastName

I have created the following poco classes:

public class User {
    public virtual int UserID { get; set; }
    public virtual string UserName { get; set; }
    public virtual string Password { get; set; }
    public virtual UserDetails Details { get; set; }
}

public class UserDetails {
    public virtual int UserID { get; set; }
    public virtual User User { get; set; }
    public virtual string FirstName { get; set; }
    public virtual string LastName { get; set; }

    public UserDetails() {
    }

    public UserDetails(User user) {
        User = user;
    }
}

Which are fluently mapped (please note the xml mapping is very similar and if all you know is the xml mapping then I would still appreciate you guidance):

public class UserMap : ClassMap<User> {
    public UserMap() {
        Table("Users");
        Id(x => x.UserID);
        Map(x => x.UserName);
        Map(x => x.Password);
        HasOne(x => x.Details)
            .Constrained()
            .Cascade.All();
    }
}

public class UserDetailsMap : ClassMap<UserDetails> {
    public UserDetailsMap() {
        Table("UsersDetails");
        Id(x => x.UserID)
            .GeneratedBy.Foreign("User");
        HasOne(x => x.User)
            .Constrained();
        Map(x => x.FirstName);
        Map(x => x.LastName);
    }
}

Everything displays correctly but if I say:

var user = new User() { UserName = "Test", Password = "Test" };
user.Details = new UserDetails(user) { FirstName = "Test", LastName = "Test" };
session.Save(user);

I get the error:

"NHibernate.Id.IdentifierGenerationException: null id generated for: UserDetails."

I'd really appreciate it if someone could show me what I've done wrong. Thanks