Nhibernate Join 2 QueryOver - nhibernate

Question

I can't find how to join two different QueryOver, group by and perform a substraction in the select.
Say you have :
public class EntityA
{
public virtual int Id;
public virtual string Reference;
public virtual int Quantity;
[Some properties]
}
public class EntityB
{
public virtual int Id;
public virtual int EntityAId;
[Some properties]
}
If i translate my query in pseudo-SQL, i would like to have :
SELECT A.Id, A.Reference, A.Quantity - COALESCE(DERIV_B.TOTAL, 0)
FROM EntityA A
LEFT JOIN (
SELECT B.EntityAId, COUNT(B.Id) AS TOTAL
FROM EntityB B
GROUP BY B.EntityAId) DERIV_B
ON A.Id = DERIV_B.EntityAId
WHERE (A.Quantity - COALESCE(DERIV_B.TOTAL, 0)) >= 0
I can have the subquery on EntityB via QueryOver, but i can't join on EntityA :
var entitiesB = GetCurrentSession().QueryOver<EntityB>().SelectList(select => select.SelectGroup(x => x.EntityAId).SelectCount(x => x.Id));
var entitiesA = GetCurrentSession().QueryOver<EntityA>(). ???
I tried to store the entitiesB in an alias and the perform a JoinAlias on it but i have an exception because it can't retrieve my alias.
Do you have any solution ?
I don't want to create a reference between these two entites.

Answer 1

Short answer is not , you can't do QueryOver if your entities are not connected through model.
One solution would be to use NHibernate.Linq and subqueries
var session = GetCurrentSession();
var entityBQuery = session.Query<EntityB>();
var entityAQuery = session.Query<EntityA>()
.Select(eA=>new { Id = eA.Id,
Description = eA.Description,
Quantity = eA.Quantity - entityBQuery.Where(eb=>eb.EntityAId = eA.Id).Count()
});