public virtual void TestGroupBySum()
{
var list = db.Orders.Where(o => o.CustomerID == "ALFKI").GroupBy(o => o.CustomerID).Select(g => g.Sum(o => (o.CustomerID == "ALFKI" ? 1 : 1))).ToList();
AssertValue(1, list.Count);
AssertValue(6, list[0]);
}