How do I use NHibernateMaps for foreign keys?

I have the following class:

public class Element
    {
        public Guid Id { get; set; }
        public ElementRouting route { get; set; }
    }

An Element can be routed to another element in a parent-child - like relationship represented by the following class:

public class ElementRouting
    {
        public Guid Id { get; set; }
        public开发者_运维知识库 Element ParentElement { get; set;}
        public Element ChildElement { get; set; }
    }

My NHibernateMap for Element looks like this:

public class ElementMapping : ClassMap<Element>
{
    public ElementMapping()
    {
        Id(x => x.Id).GeneratedBy.Assigned();
        Not.SelectBeforeUpdate();
        References(x => x.Route).Column("ElementRoutingId").Nullable();

        Table("Elements");
    }
}

...and the map for ElementRouting is as follows:

public class ElementRoutingMapping : ClassMap<ElementRouting>
{
    public ElementRoutingMapping()
    {
        Id(x => x.Id).GeneratedBy.Assigned();
        Map(x => x.ChildElement).Column("ElementId").Nullable();
        Map(x => x.ParentElement).Column("ElementId").Nullable();

        Table("ElementRoutings");
    }
}

At the point that I session.Flush() I have an Element object with null for its property ElementRouting. I get a HibernateException (An exception occurred when executing batch queries) with this InnerException detail:

{"The UPDATE statement conflicted with the FOREIGN KEY constraint "FK54CBB3751C1EAB64". The conflict occurred in database "myDataBaseName", table "dbo.ElementRoutings", column 'Id'.}

Can anyone explain to me what I am doing wrong?

---

For the Element Mapping's reference to the route, used:

References(x => x.TonsRoute).Column("TonsRouteId").Nullable().Cascade.All();

which does not cause a conflict.