Using a reference as id in fluentnhibernate

I have a child table containing an id to the parent. This is a one to one mapping, but the child table might be missing values. I'm having problems mapping this without getting an error though... I've tried several things; mapping the same column, having distinct properties etc..

Parent table
  int id

Child table
  int parentid

Parent class
  int id

Child class
  Parent parent // note I'm referencing parent, not using an int id..

Mapping

Id(x => x.Parent)
  .Column("parentid"); // fails

Id(x => x.Parent.Id)
  .Column("parentid"); // fails

References(x => x.Parent)
  .Column("parentid"); // fails - missing id

// Adding an id field in addition to parent for
// child class (id is then the same as parent.id)
// fails on save
Id( x => x.Id ) 
  .Column("parentid");
References(x => x.Parent)
  .Column("parentid");

I would like the child class not to have a distinct Id field, but rather only a reference to parent as there can never be a child without a parent. In the database however, I want to just store the parent's id.

Any ideas how I might do this?