NHibernate QueryOver Sum within JoinQueryOver - nhibernate

Question

Though I was reading through the NHibernate Cookbook and all available forum-posts up and down, I'm still not able to get this simple query done:
I have users with everyone having one account. Each account hast a balance.
The classes look like that:
public class User
{
public virtual int Id { get; set; }
public virtual Account Account { get; set; }
public virtual bool Active { get; set; }
}
public class Account
{
public virtual int Id { get; set; }
public virtual double Balance { get; set; }
}
Now I would like to sum the balance of all active users. Nothing more...
In plain SQL it is quite easy:
SELECT SUM(a.Balance)
FROM User u
INNER JOIN Account a
ON u.Account_id = a.Id
WHERE u.Active = 'true'
I don't have any I idea, how I could solve that with the new QueryOver-Api from NHibernate 3. Could you please provide a code-example?
Thank you in advance!
Daniel Lang
EDIT
I know, that with NHibernate Linq it is very easy too, but I would like to solve it using QueryOver... Here is the working Linq-Example:
var result = Session.Query<User>()
.Where(x => x.Active)
.Sum(x => x.Account.Balance)
SOLUTION
Thanks to AlexCuse I could find the final solution (he was very very close) - here is the full code:
User userAlias = null;
Account accountAlias = null;
session.QueryOver<User>(() => userAlias)
.JoinAlias(() => userAlias.Account, () => accountAlias)
.Where(() => userAlias.Active)
.Select(Projections.Sum<Account>(acct => accountAlias.Balance))
.SingleOrDefault<double>()

Answer 1

Have you tried something like this?
session.QueryOver<User>(() => userAlias)
.JoinAlias(() => userAlias.Account, () => accountAlias)
.Where(() => userAlias.Active)
.Select(Projections.Sum<Account>(acct => acct.Balance))
.UnderlyingCriteria.UniqueResult()
I'm not sure what the UniqueResult equivalent is in the QueryOver API, so had to go through the underlying criteria.

Answer 2

You wrote in your answer:
User userAlias = null;
Account accountAlias = null;
session.QueryOver<User>(() => userAlias)
.JoinAlias(() => userAlias.Account, () => accountAlias)
.Where(() => userAlias.Active)
.Select(Projections.Sum<Account>(acct => accountAlias.Balance))
.SingleOrDefault<double>()
You don't need to have an alias for the generic type. It could be:
Account accountAlias = null;
session.QueryOver<User>()
.JoinAlias(user => user.Account, () => accountAlias)
.Where(user => user.Active)
.Select(Projections.Sum<Account>(acct => accountAlias.Balance))
.SingleOrDefault<double>()