NHibernate using wrong key in queries, ObjectNotFoundByUniqueKeyException thrown - nhibernate

Question

Using NHibernate 5.2, I'm getting an ObjectNotFoundByUniqueKeyException due to erroneous queries being executed with an invalid key when I do the following:
var session = sessionFactory.OpenSession();
var employee = new Employee(){
UserId = "joe",
Certifications = new List<Certification>()
};
employee.Certifications.Add(new Certification() { Employee = employee});
var id = session.Save(employee);
session.Flush();
session.Clear();
var emp = session.Get<Employee>(id);
foreach(var e in emp.Certifications)
{
Console.WriteLine(e.Id);
}
Queries executed
NHibernate: INSERT INTO Employee (UserId) VALUES (#p0); select SCOPE_IDENTITY();#p0 = 'joe' [Type: String (4000:0:0)]
NHibernate: INSERT INTO Certification (UserId) VALUES (#p0); select SCOPE_IDENTITY();#p0 = 'joe' [Type: String (4000:0:0)]
NHibernate: SELECT userquery_0_.Id as id1_0_0_, userquery_0_.UserId as userid2_0_0_ FROM Employee userquery_0_ WHERE userquery_0_.Id=#p0;#p0 = 1 [Type: Int32 (0:0:0)]
NHibernate: SELECT certificat0_.UserId as userid2_1_1_, certificat0_.Id as id1_1_1_, certificat0_.Id as id1_1_0_, certificat0_.UserId as userid2_1_0_ FROM Certification certificat0_ WHERE certificat0_.UserId=#p0;#p0 = 1 [Type: Int32 (0:0:0)]
NHibernate: SELECT userquery_0_.Id as id1_0_0_, userquery_0_.UserId as userid2_0_0_ FROM Employee userquery_0_ WHERE userquery_0_.UserId=#p0;#p0 = '1' [Type: String (4000:0:0)]
I'm expecting #p0 = 'joe' in the final two queries, not 1 and '1'. Can anyone see a problem with the mappings below that would explain this behavior?
Classes / Mappings
public class Employee
{
public virtual int Id { get; set; }
public virtual string UserId { get; set; }
public virtual ICollection<Certification> Certifications { get; set; }
}
public class EmployeeMap : ClassMap<Employee>
{
public EmployeeMap()
{
Table("Employee");
Id(x => x.Id);
Map(x => x.UserId);
HasMany(x => x.Certifications)
.KeyColumn("UserId")
.Cascade.All();
}
}
public class Certification
{
public virtual int Id { get; set; }
public virtual Employee Employee { get; set; }
}
public class CertificationMap : ClassMap<Certification>
{
public CertificationMap()
{
Table("Certification");
Id(x => x.Id);
References(x => x.Employee)
.Column("UserId")
.PropertyRef(x => x.UserId);
}
}

Answer 1

Collection is not using primary key of Employee, it is another property instead - PropertyRef. And we must inform one-to-many mapping as well as we do for many-to-one
HasMany(x => x.Certifications)
.KeyColumn("UserId")
.Cascade.All()
// use property, not the ID, when searching for my items
.PropertyRef("UserId")
;
A fact, that this mapping was missing, was causing the ID to be used (the magic 1) when NHibernate was loading collection