How to do a one-to-one relationship with null in NHibernate?

Question

I have two NHibernate entities with a one-to-one relationship. Let's call them Dog and Owner.

public class Dog 
{
    public virtual int Id { get;set; }
    public virtual Owner Owner { get; set; }
}

public class Owner 
{
    public virtual int Id { get; set; }
    public virtual Dog Dog { get; set; }
}

And Dog may have one or zero owners. The Fluent/NHibernate mappings are as so:

public class DogMap : ClassMap<Dog> 
{
    public DogMap() 
    {
        Table("Dogs");

        Id(x => x.Id);

        HasOne( x=> x.Owner)
            .Fetch.Join()
            .Not.LazyLoad();
    }
}

public class OwnerMap : ClassMap<Owner> 
{
    public OwnerMap() 
    {
        Table("Owners");

        // Owners share the same primary-key as dogs
        Id(x => x.Id).GeneratedBy.Foreign("Dog"); 

        References( x => x.Dog)
            .Unique()
            .Not.LazyLoad();
   }
}

Now I simply wish to select Dogs which don't have an Owner.

I assumed the query would be:

Owner owner = null;
var ownerlessDogs = session
    .QueryOver<Dog>()
    .Left.JoinAlias(x => x.Owner, () => owner)
    // Restrict on alias
    .WhereRestrictionOn(() => owner).IsNull
    .List();

But as you may have guessed, this does not work. It throws 'Object reference not set to ...'.

If I try,

var ownerlessDogs = session
    .QueryOver<Dog>()
    .Left.JoinAlias(x => x.Owner, () => owner)
    // Restrict on property of root object
    .WhereRestrictionOn(x => x.Owner).IsNull
    .List();

It generates basically generates

SELECT {{relevant columns}}
FROM Dogs dogs
LEFT OUTER JOIN OWNERS owners
WHERE dogs.Id IS NULL

Almost right, but it is filtering on the primary-key of dogs, rather than the dogs foreign-key on owners.

Answer 1

With some help from an old post in the Hibernate forum I found a solution:

public class DogMap : ClassMap<Dog> {

    public DogMap() {
        Table("Dogs");
        Id(x => x.Id);

        HasOne( x=> x.Owner)
          .Fetch.Join();
    }
}

public class OwnerMap : ClassMap<Owner> {

   public OwnerMap() {

   Table("Owners");

   //owners share the same primary-key as dogs
   Id(x => x.Id).GeneratedBy.Foreign("Dog"); 

   //both sides should be mapped as HasOne
   HasOne( x => x.Dog)
     .Constrained()
     .Fetch.Join();
   }
}

And the working query is

var ownerlessDogs = session
  .QueryOver<Dog>()
  .Left.JoinAlias(x => x.Owner, () => owner)
  //this is the trick, restrict on Id
  .WhereRestrictionOn( x => x.Owner.Id ).IsNull
  .List();