Polymorphic count with NHibernate

Although the feature is powerful, you can find a small problem. How to count entities returned by the query without getting'em all to the memory. Setting a simple projection *Projections.RowCount()* will not work. Why? 'Cause it's gonna 
        query each table with *COUNT* demanding at the same time that *IDataReader* should contain only one, unique result. It won't happen, and all you'll be left with it'll be a nice exception. So, is it possible to count entities in a polymorphic way? Yes! You can define in a extension method and use it every time you need a polymorphic count.

[sourcecode language="csharp" light="true"]
private static int GetPolymorphicCount(ISession s, Type countedType)
{
    var factory = s.GetSessionImplementation().Factory;
    var implementors = factory.GetImplementors(countedType.FullName);

    return implementors
        .Select(i => s.CreateCriteria(i)
                .SetProjection(Projections.RowCount())
                .FutureValue<int>())
        .ToArray() // to eagerly create all the future values for counting
        .Aggregate(0, (count, v) => count + v.Value); // sum up counts
}