Fluent Nhibernate reference object by id, and not by object

public class Foo 
{
   public virtual long ID { get; set; }
   public virtual string Name { get; set; }
   public virtual Foo2 Foo2 { get; set; }
}

public class FooMap: ClassMap<Foo>
{
   public FooMap()
   {
      id(x=>x.ID);
      Map(x=>x.Name);
      
      References(x=>x.Foo2).Cascade.All;
   }
}

public class Foo 
{
   public virtual long ID { get; set; }
   public virtual string Name { get; set; }
   public virtual long Foo2ID { get; set; }
}