NHibernate Cascaded delete not working on one-to-many association - nhibernate

Question

I am trying to delete an object and cascade the delete to the child objects in a one-to-many association. I think that I have done everything correctly for this to work. However, when I run my test, NHibernate attempts to insert a null value into the foreign key column of the child table rather than deleting the items.
From my parent mapping (Carrier):
<set name="Drivers" access="field.camelcase-underscore">
<key column="CarrierId"/>
<one-to-many class="Vehicle"/>
</set>
From my child mapping (Vehicle):
<many-to-one name="Carrier" class="Carrier" column="CarrierId" not-null="true"/>
My test:
[Test]
public void Can_delete_a_carrier_and_associated_vehicles() {
object id;
var carrier = new Carrier { BusinessRef = 759540, Name = "Carrier1" };
var vehicle = new Vehicle { Carrier = carrier, BusinessRef = "FOOBAR", VehicleType = VehicleType.Trailer };
using (var txn = session.BeginTransaction()) {
id = session.Save(carrier);
session.Save(vehicle);
txn.Commit();
}
session.Clear();
using (var txn = session.BeginTransaction()) {
var fromDb = session.Get<Carrier>(id);
Assert.IsNotNull(fromDb);
Assert.AreEqual("FOOBAR", fromDb.Vehicles.First().BusinessRef);
session.Delete(fromDb);
txn.Commit();
}
}
The generated SQL:
INSERT INTO Carriers (...) VALUES (...); select last_insert_rowid();#p0 = 'WSH', #p1 = 759540, #p2 = False
INSERT INTO Vehicles (...) VALUES (...); select last_insert_rowid();#p0 = 2, #p1 = 'FOOBAR', #p2 = 4
SELECT carrier0_.Id, ... FROM Carriers carrier0_ WHERE carrier0_.Id=#p0;#p0 = 4
SELECT vehicles0_.CarrierId as CarrierId1_, ... FROM Vehicles vehicles0_ WHERE vehicles0_.CarrierId=#p0;#p0 = 4
UPDATE Vehicles SET CarrierId = null WHERE CarrierId = #p0;#p0 = 4
It's the line in bold that is causing the test to fail because I have a not null constraint on carrier (see vehicle mapping).
This is what I don't understand, if I have a not-null constraint, why does NHibernate try and insert null into the column.
So what do I need to do to ensure that deleting a carrier, deletes all vehicles?
Thanks,
Ben

Answer 1

After all this, the problem ended up being a typo in one of the other sets defined on the parent object. It was only through trying a few more specific tests that I found I was trying to cast a collection to the wrong type - doh!
So basically, if you use the mapping above then the deletes will cascade (providing you don't make silly typos :))