Linq's flexibility

If you find incessant aliasing a bit too distracting...

var py =
   from p in s.Query<Product>()

   from o in p.Orders
   group o by new { p.ProductId } into grp
   select new { grp.Key.ProductId, Count = grp.Count() };

...you can do it with lambda approach:

var py =
 from p in s.Query<Product>()
  .SelectMany(_ => _.Orders)
  .GroupBy(_ => new { _.Product.ProductId })
 select new { p.Key.ProductId, Count = p.Count() };