Count Elements Conditionally In HQL

I have an HQL query that returns projections of an entity including a count of the a certain child collection.

select t.Id as TagId, t.Name as Name, count(elements(t.Documents)) as Count 
from Tag t
group by t.Id, t.Name, t.User
having t.User.Id = :userId"

It works except I need to place a condition on the count. I only want to count Documents where Trashed = false for the document.

I seem to have a mental block on how to do this. I am using NHibernate 3.0. I would be equally happy with ICriteria or Linq query.

UPDATE: Here is the fully correct开发者_运维技巧 query:

select t.Id as TagId, t.Name as Name, sum(case when d.Trashed = false then 1 else 0 end) as Count 
from Tag t
left join t.Documents d
group by t.Id, t.Name, t.User
having t.User.Id = :userId and sum(case when d.Trashed = false then 1 else 0 end) > 0

IIRC, something like this should work (where d is an alias to the Documents collection):

sum(case when d.Trashed = false then 1 else 0 end) as Count