Fluent NHibernate - 1:1 Relationship with Nullable Field - nhibernate

Question

Given the following model, I want to be able to enter a new WriteOffApprovalUser and have the Employee field be null. This is a 1:1 or null relationship.
public class WriteOffApprovalUser
{
public virtual string UserName { get; set; }
public virtual Employee Employee { get; set; }
}
public class Employee
{
public virtual string EmployeeID { get; set; }
public virtual string EmployeeStatusCode { get; set; }
public virtual string FirstName { get; set; }
public virtual string LastName { get; set; }
public virtual string JobTitle { get; set; }
public virtual string Division { get; set; }
public virtual string Department { get; set; }
public virtual string Location { get; set; }
public virtual string City { get; set; }
public virtual string DeskLocation { get; set; }
public virtual string MailID { get; set; }
public virtual string Phone { get; set; }
public virtual string PreferredName { get; set; }
public virtual string Fax { get; set; }
public virtual string SecCode { get; set; }
public virtual string SupervisorID { get; set; }
public virtual string UserId { get; set; }
}
Class Maps
public class WriteOffApprovalUserMap : ClassMap<WriteOffApprovalUser>
{
public WriteOffApprovalUserMap()
{
Table("WRITEOFF_APPROVAL_USER");
Id(x => x.UserName).Column("USER_NAME");
//Map(x => x.Employee).Nullable();
HasOne(x => x.Employee)
.Class<Employee>()
.Constrained()
.Cascade.SaveUpdate()
.PropertyRef("UserId");
}
}
public class EmployeeMap : ClassMap<Employee>
{
public EmployeeMap()
{
Table("ADP_EMPLOYEE");
Id(x => x.EmployeeID).Column("EMPLID").GeneratedBy.Native("");
Map(x => x.FirstName).Column("FIRST_NAME");
Map(x => x.LastName).Column("LAST_NAME");
Map(x => x.PreferredName).Column("PREFERRED_NAME");
Map(x => x.UserId).Column("USER_ID");
}
}
Query/Save
using (var session = SessionProvider.GetSession())
{
using (var tx = session.BeginTransaction())
{
var user = new WriteOffApprovalUser() { UserName = "SAMSTR" };
session.Save(user);
tx.Commit();
}
}
This complains that Employee is null. How do I specify that Employee can be null?
Also, do all Id fields have to be integral? A lot of the keys on our tables are strings.

Answer 1

First off all if you put it as 1:1 it shouldn't be NULL because it's not correct design.
But here is an example of how to do so:
1) One way
HasOne(x => x.Employee)
.Class<Employee>().Nullable().NotFound.Ignore().PropertyRef("UserId");
2) Second way
References(x => x.Category).Column("UserId").Nullable().NotFound.Ignore();