nhibernate group by select column based on restrictions - nhibernate

Question

I have what seems to be a rather simple problem, that I cannot figure out right now.
I have a table like so:
id | type | value | comment | date
1 1 22 test dec 2nd
2 1 23 foo dec 4th
3 2 2 bar dec 1st
Based on the model
class MyClass
public virtual long Id { get; set;}
public virtual long Type { get; set;}
public virtual long Value { get; set;}
public virtual string comment { get; set;}
public virtual DateTime Date { get; set;}
I need to group by type and select the row having the most recent date.
(That is, fetch rows with ID 2 and ID 3).
Can someone provide a Criteria with explanation of how to do this ?

Answer 1

two roundtrips, 1 to get the type/date of the desired rows and 1 combined to get each row. FutureValue<> will combine each Query in the loop to one roundtrip and the select converts the FutureValues to the actual values.
class TypeDate
{
public long Type { get; set; }
public DateTime Date { get; set; }
}
var groups = session.CreateCriteria<MyClass>()
.SetProjection(Projections.ProjectionList()
.Add(Projections.GroupProperty("Type"), "Type")
.Add(Projections.Max("Date"), "Date"))
.SetResultTransForm(Transformers.AliasToBean<TypeDate>());
.List<TypeDate>();
List<IFutureValue<MyClass>> futures = new List<IFutureValue<MyClass>>(groups.Count);
foreach (var group in groups)
{
futures.Add(session.CreateCriteria<MyClass>()
.Add(Restrictions.Eq("Type", group.Type))
.Add(Restrictions.Eq("Date", group.Date))
.FutureValue<MyClass>());
}
IEnumerable<MyClass> results = futures.Select(future => future.Value).ToList();