public async Task<ActionResult> Create() {
     var order = new Order() { Number = Db.Orders.Any() ? Db.Orders.Max(x => x.Number) + 1 : 1 };
     await Db.AddAsync(order);
     return Get(order.Id);
 }
 public async Task AddAsync(Order order) {
     await Task.Run(() => Orders.Add(order));
 }