Linq to NHibernate ThenFetch多个属性

// Lots of stuff omitted for brevity; these are all virtual properties and there
// are other properties which aren't shown on all classes.
class A {
    B b;
    C c;
    DateTime timestamp;
}
class B {
    X x;
    Y y;
}
class X {
    int id;
}
class C { }
class Y { }

or to put it more simply,
a = {
   b: {
      x { id: int },
      y: { }
   },
   c: { },
   timestamp: DateTime
}      

A

B

C

X

Y

ILookup<B, A> GetData(List<int> ids) {
    using (ISession session = OpenSession()) {
        var query = from a in session.Query<A>()
                    where ids.Contains(a.b.x.id)
                    orderby A.timestamp descending
                    select a;

        query = query
            .Fetch(a => a.b)
            .ThenFetch(b => b.x)
            .Fetch(a => a.b)
            .ThenFetch(b => b.y)
            .Fetch(a => a.c);

       return query.ToLookup(a => a.b);
   }
}

group by

select  /* snipped - every mapped field from a0, b1, x2, b3, y4, c5 - but not b6 */
from     [A] a0
         left outer join [B] b1
           on a0.B_id = b1.BId
         left outer join [X] x2
           on b1.X_id = x2.XId
         left outer join [B] b3
           on a0.B_id = b3.BId
         left outer join [Y] y4
           on b3.Y_id = y4.YId
         left outer join [C] c5
           on a0.C_id = c5.CId,
         [B] b6
where    a0.B_id = b6.BId
         and (b6.X_id in (1, 2, 3, 4, 5))
order by a0.timestamp desc

a.b

b1

b3

b6

.Fetch

a.b