EntityA.B refers to EntityB

EntityB.Item refers to IItem (persistent interface type)

from a in Session.Query.All<EntityA>()
where a.User == myUser
group a by a.B into g
select new { g.Key.Item.Name, Count = g.Count() };

Throws KeyNotFound exception.

If you remove g.Key.Item.Name from the select list (and replace it with something like g.Key.SomeValueField), it works.

In the meantime, I made a workaround to select g.Key (then access what would be g.Key.Item.Name through code), and added a prefetch for what would be g.Key.Item.Name so that there isn't a second roundtrip to DB.