Write-only Master-Detail relationships in DataService?
public class OrderService : DataService< OrdersDataContext >
{
    // This method is called only once to initialize service-wide policies.
    public static void InitializeService(DataServiceConfiguration config)
    {
        config.DataServiceBehavior.MaxProtocolVersion = DataServiceProtocolVersion.V2;
        config.SetEntitySetAccessRule("Orders", EntitySetRights.WriteAppend);
        config.SetEntitySetAccessRule("OrderItems", EntitySetRights.WriteAppend);

        config.UseVerboseErrors = true;
    }
}

OrderServiceDataContext orderService = new OrderServiceDataContext(new Uri("http://localhost/OrderService.svc"));
Order order = new Order();
orderService.AddToOrders(order);
foreach(CartItem cartItem in ShoppingCart)
{
    OrderItem orderItem = new OrderItem(){
      ProductId = cartItem.Id,
      Quantity = cartItem.Quantity
    };
    orderService.AddToOrderItems(orderItem);
    order.OrderItems.Add(orderItem);
    orderService.AddLink(order, "OrderItems", orderItem);
}

orderService.SaveChanges(SaveChangesOptions.Batch);