var ordersByProduct =
  (from c in customers
    from o in c.Orders
    select o)
  .ToLookup(o => o.IdProduct);

Console.WriteLine( "\n\nPoet objednvek pro produkt 1: {0}\n",
                  ordersByProduct[1].Count());

foreach (var product in ordersByProduct) {
  Console.WriteLine("Produkt: {0}", product.Key);
  foreach(var order in product) {
    Console.WriteLine(" {0}", order);
  }
}
