NHibernate and JoinAlias throw exception - nhibernate

Question

I have query in HQL which works good:
var x =_session.CreateQuery("SELECT r FROM NHFolder f JOIN f.DocumentComputedRights r WHERE f.Id = " + rightsHolder.Id + " AND r.OrganisationalUnit.Id=" + person.Id);
var right = x.UniqueResult<NHDocumentComputedRight>();
Basically I receive NHDocumentComputedRight instance.
I've tried to implement the same query in QueryOver. I did this:
var right = _session.QueryOver<NHFolder>().JoinAlias(b => b.DocumentComputedRights, () => cp).Where(h => h.Id == rightsHolder.Id && cp.OrganisationalUnit.Id == person.Id)
.Select(u => cp).List<NHDocumentComputedRight>();
But I get null reference exception.
How can I implement this query in QueryOver?
Update (added mappings) - NHibernate 3.2:
public class FolderMapping: ClassMapping<NHFolder>
{
public FolderMapping()
{
Table("Folders");
Id(x => x.Id, map =>
{
map.Generator(IdGeneratorSelector.CreateGenerator());
});
//more not important properties...
Set(x => x.DocumentComputedRights, v =>
{
v.Table("DocumentComputedRightsFolder");
v.Cascade(Cascade.All | Cascade.DeleteOrphans);
v.Fetch(CollectionFetchMode.Subselect);
v.Lazy(CollectionLazy.Lazy);
}, h => h.ManyToMany());
Version(x => x.Version, map => map.Generated(VersionGeneration.Never));
}
}
public class DocumentComputedRightMapping : ClassMapping<NHDocumentComputedRight>
{
public DocumentComputedRightMapping()
{
Table("DocumentComputedRights");
Id(x => x.Id, map =>
{
map.Generator(IdGeneratorSelector.CreateGenerator());
});
//more not important properties...
ManyToOne(x => x.OrganisationalUnit, map =>
{
map.Column("OrganisationalUnit");
map.NotNullable(false);
map.Cascade(Cascade.None);
});
}
}
public class OrganisationUnitMapping : ClassMapping<NHOrganisationalUnit>
{
public OrganisationUnitMapping()
{
Table("OrganisationalUnits");
Id(x => x.Id, map =>
{
map.Generator(IdGeneratorSelector.CreateGenerator());
});
//more not important properties...
}
}
Thanks

Answer 1

AFAIK criteria/queryOver can only return the entity it was created for (NHFolder in your example) or columns which are set to entity with aliastobean. you could do a correlated subquery instead.
var subquery = QueryOver.Of<NHFolder>()
.JoinAlias(b => b.DocumentComputedRights, () => cp)
.Where(h => h.Id == rightsHolder.Id && cp.OrganisationalUnit.Id == person.Id)
.Select(u => cp.Id);
var right = _session.QueryOver<NHDocumentComputedRight>()
.WithSubquery.Where(r => r.Id).Eq(subquery)
.SingleOrDefault<NHDocumentComputedRight>();

Answer 2

I think you have a problem with the select statement, have you tried something like this:
var right = _session.QueryOver<NHFolder>()
.JoinAlias(b => b.DocumentComputedRights, () => cp)
.Select(x => x.DocumentComputedRights)
.Where(h => h.Id == rightsHolder.Id && cp.OrganisationalUnit.Id == person.Id)
.List<NHDocumentComputedRight>();
This is what is working for me so it should work in you case as well.
I would guess that the main reason behind the problem is the lack of proper overload on the Select method. In reality you would like to write it like this:
.JoinAlias(b => b.DocumentComputedRights, () => cp)
.Select(() => cp)
but the Expression<Func<object>> is not there. Hopefully it's going to be included in the next version.