projection of type average(datediff) always returns 0.0

This is very puzzling; the generated SQL is perfectly good, and I get correct results when running it manually.

However, somewhere in the transformation process the 'AverageTime' field is set to 0.0 instead of the correct result.

my query:

var query = Session.CreateCriteria<Employee>()
                .Add(Expression.In("Department", departments.ToArray()))  // departmentsContains(employee))
                .Add(Expression.Ge("TimeOut", startTime)) // TimeOut >= startTime 
                .Add(Expression.Le("TimeOut", endTime))   // TimeOut <= endTime
                .SetProjection(Projections.Alias(Projections.GroupProperty("开发者_如何学CDepartment.Id"), "Id")
                    , Projections.Alias(Projections.Count("Id"), "EmpCount")      //total emps
                    , Projections.Avg(  //average of..
                        Projections.SqlProjection("datediff(ss, {alias}.TimeIn ,{alias}.TimeOut) as AverageTime", new[] { "AverageTime" }, new[] { NHibernateUtil.Double })      // waiting time
                    )
                )
                .SetResultTransformer(NHibernate.Transform.Transformers.AliasToBean<EmpsForStatistics>())
                .List<EmpsForStatistics>();


private class EmpsForStatistics
        {
            public int DepartmentId { get; set; }
            public int EmpCount { get; set; }
            public double AverageTime { get; set; }
        }

the generated query is correct:

SELECT this_.Department_id as y0_, count(this_.Id) as y1_, avg(cast(datediff(ss, this_.TimeOut ,this_.TimeIn) as DOUBLE PRECISION)) as y2_ 
FROM nHibernate_test.dbo.[Employees] this_ 
    WHERE this_.Department_id in (4004, 4005, 4006) 
    and this_.TimeOut >= '06/07/2011 08:27:58' and this_.TimeOut <= '06/07/2011 11:27:58' 
    GROUP BY this_.Department_id;

p.s. obviously measuring employees' average time is just for the example's sake. My real query is for other entities..

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OK I'm totaly stupid. Forgot to add an Alias() to the Avg projection... duhh