NHibernate 5.1 ManyToOne - delete orphan not working as expected

I have following entities:

public class Person {
    public int Id {get;set;}
    public String Name {get;set;}
    public Passport Passport {get;set;}
}

public class Passport {
    public int Id {get;set;}
    public String PassportNo {get;set;}
    // ... other properties
}

and following mapping (for Person):

ManyToOne(x => x.Passport, m =>
    {
        m.Column("PassportId");
        m.Lazy(LazyRelation.NoLazy);
        m.Unique(true);
        m.Cascade(Cascade.All | Cascade.DeleteOrphans);
    });

DB schema:

Table Person:

Id | Name | PassportId

Table Passport:

Id | PassportNo | other props.

As we can see, Person has it's passport, but passport has no idea about its owner, which is behaviour I want. There are 2 more assumptions:

• Person can have only 1 passport at a time
• Passport can NOT exists without person

Problem is, when I assign new Passport to a Person, the old Passport remains in DB.

person.Passport = new Passport() { PassportNo = "AB 123456" };
// ...
session.Flush();
session.Commit();

SQL queries that are generated are INSERT and UPDATE (inserts new Passport, and updates Person with new Passport) - however there is no DELETE on orphaned, old passport.

The workaround I found is to set current Passport to null, call session.Flush(), and assign new Passport, like this:

person.Passport = null;
session.Flush();
person.Passport = new Passport() { PassportNo = "AB 123456" };
// ...
session.Flush();
session.Commit();

however IMO it is a hacky solution.

So, summing it up:

• am I missing something in my mapping?
• is above behaviour a bug in NH?
• can it be solved without hacky Flush()?

After long searching for solution, I found that there is a Join mapping, which perfectly fits above scenario.

Hope it helps someone.

More info here: http://notherdev.blogspot.com/2012/01/mapping-by-code-join.html