How do I return only one specific item from hasmany relationship - nhibernate

Question

Though I have benefited from the collective wisdom of this site many times, this is my first question here.
I have, say, three classes like so:
public class ClassA
{
public ClassA() {}
public virtual IList<ClassB> ClassBs { get; set; }
}
public class ClassB
{
public ClassB() {}
public virtual DateTime StartDate { get; set; }
public virtual DateTime? EndDate { get; set; }
public virtual ClassC SomeObject { get; set; }
}
public class ClassC
{
public ClassC() {}
public virtual string Name { get; set; }
}
I am using NHibernate (3.3.1.4) and FluentNHibernate (1.3.0.733), and the mapping files are:
public Class ClassAMap : ClassMap<ClassA>
{
HasMany(x => x.ClassBs);
}
public Class ClassBMap : ClassMap<ClassB>
{
Map(x => x.StartDate).Not.Nullable();
Map(x => x.EndDate).Nullable();
References(x => x.SomeData).Not.Nullable();
}
public Class ClassCMap : ClassMap<ClassC>
{
Map(x => x.Name).Not.Nullable();
}
There are also IDs and versioning but I somehow think they are irrelevant.
What I want to do is:
select all "SomeObject"s from ClassBs of all "ClassA"s which have their "EndDate"s null and which have the most current StartDate in their group.
I tried some juggling with QueryOver but the most I could get was an IList<IList<ClassB>> which is really far from what I want to accomplish.
Edit:
(I think) Following code accomplishes the task with Linq. But this code requires all records from the DB. This may not be a problem for ClassBs with up to 3 records or so per ClassA but for ClassBs with hundreds of records this means getting all those records from DB to use just one record from ClassBs of each ClassA in the DB.
IList<ClassC> classCLs = new List<ClassC>();
ClassB latest = null;
foreach (
IList<ClassB> classBLs in
Session.QueryOver<ClassA>()
.Select(c => c.ClassBs).List<IList<ClassB>>()
) {
latest = classBLs.Where(cB => cB.EndDate == null).Aggregate((curr, next) => next.StartDate > curr.StartDate ? next : curr);
if (latest != null && !classCLs.Contains(latest.SomeObject)) {
classCLs.Add(latest.SomeObject);
}
}

Answer 1

Assuming your ClassA has an Id property, maybe this, involving a subquery, can be of some help :
ClassA aAlias = null;
ClassB bAliasMax = null, bAlias = null;
var subQuery = QueryOver.Of<ClassA>().JoinAlias(a => a.ClassBs, () => bAliasMax)
.Where(Restrictions.On(() => bAliasMax.EndDate).IsNotNull)
.Where(a=>a.Id==aAlias.Id)
.Select(Projections.Max(() => bAliasMax.StartDate));
var result =
_laSession.QueryOver(() => aAlias)
.JoinAlias(a => a.ClassBs, () => bAlias)
.WithSubquery.WhereProperty(() => bAlias.StartDate).Eq(subQuery)
.Select(Projections.Property(() => bAlias.SomeObject)) // suggested adding from question's author
.List<ClassC>(); // see above
I guess there are more effective answers, which would involve grouping.