How do I "turn on" cascading saves using AutoMap Persistence Model with Fluent NHibernate?

As in:

I Save the Person and the Arm should also be saved. Currently I get

"object references an unsaved transient instance - save the transient instance before flushing"

public class Person : DomainEntity
{
  public virtual Arm LeftArm { get; set; }
}

public class Arm : DomainEntity
{
  public virtual int Size { get; set; }
}

I found an article on this topic, but it seems to be outdated.