Summary: Parent and Child class. One to one relationship between the Parent and Child. Parent has a FK property which references the primary key of the Child. Code as follows:

  public class NHTestParent
  {
    public virtual Guid NHTestParentId { get; set; }
    public virtual Guid ChildId
    {
      get
      {
        return ChildRef.NHTestChildId;
      }
      set { }
    }
    public virtual string ParentName { get; set; }

    protected NHTestChild _childRef;
    public virtual NHTestChild ChildRef
    {
      get
      {
        if (_childRef == null)
          _childRef = new NHTestChild();
        return _childRef;
      }
      set
      {
        _childRef = value;
      }
    }
  }    

  public class NHTestChild
  {
    public virtual Guid NHTestChildId { get; set; }
    public virtual string ChildName { get; set; }
  }

With the following Fluent mappings:

Parent Mapping

  Id(x => x.NHTestParentId);
  Map(x => x.ParentName);
  Map(x => x.ChildId);
  References(x => x.ChildRef, "ChildId").Cascade.All();

Child Mapping:

  Id(x => x.NHTestChildId);
  Map(x => x.ChildName);

If I do something like (pseudo code) ...

HTestParent parent = new NHTestParent();
parent.ParentName = "Parent 1";
parent.ChildRef.ChildName = "Child 1";
nhibernateSession.SaveOrUpdate(aParent);
Commit;

... I get an error: "Invalid index 3 for this SqlParameterCollection with Count=3"

If I change the parent 'References' line as follows (i.e. provide the name of the child property I'm pointing at):

References(x => x.ChildRef, "ChildId").PropertyRef("NHTestChildId").Cascade.All();

I get the error: "Unable to resolve property: NHTestChildId" So, I tried the 'HasOne()' reference setting, as follows:

HasOne<NHTestChild>(x => x.ChildRef).ForeignKey("ChildId").Cascade.All().Fetch.Join();

This results in the save working, but the load fails to find the child. If I inspect the SQL Nhibernate produces I can see that NHibernate is assuming the Primary key of the parent is the link to the child (i.e. load join condition is "parent.NHTestParentId = child.NHTestChildId). The 'ForeignKey' specified appears to be ignored. I can set any value and no error occurs - the join just always fails and no child is returned.

I've tried a number of slight variations on the above. It seems like it should be a simple thing to achieve. Any ideas?