Error after filling data in fields?

Question

ASP.NET

The error occurs when I click on the completion of the order
PS I added the EntityFrameworkCore and created all the necessary tables

namespace WebApplication29.Data.Repository
{
    public class OrdersRepository : IAllOrders
    {
        private readonly AppDBContent appDBContent;
        private readonly ShopCart shopCart;

        public OrdersRepository(AppDBContent appDBContent, ShopCart shopCart)
        {
            this.appDBContent = appDBContent;
            this.shopCart = shopCart;
        }
        public void createOrder(Order order)
        {
            order.orderTime = DateTime.Now;
            appDBContent.Order.Add(order);

            var items = shopCart.listShopItems;

            foreach(var el in items)
            {
                var orderDetail = new OrderDetail()
                {
                    CarId = el.car.id,
                    orderID = order.id,
                    price = el.car.price
                };
                appDBContent.OrderDetail.Add(orderDetail);
            }
            appDBContent.SaveChanges();
        }
    }
}

public class OrderController : Controller
    {
        private readonly IAllOrders allOrders;
        private readonly ShopCart shopCart;

        public OrderController(IAllOrders allOrders, ShopCart shopCart)
        {
            this.allOrders = allOrders;
            this.shopCart = shopCart;
        }
        public IActionResult Checkout()
        {
            return View();
        }
    
        [HttpPost]
        public IActionResult Checkout(Order order)
        {

            shopCart.listShopItems = shopCart.getShopItems();

            if(shopCart.listShopItems.Count == 0)
            {
                ModelState.AddModelError("", "У вас должны быть товары!");
            }

            if (ModelState.IsValid)
            {
                allOrders.createOrder(order);
                return RedirectToAction("Complete");
            }

            return View(order);
        }
        public IActionResult Complete()
        {
            ViewBag.Message = "Заказ успешно обработан";
            return View();
        }
    }

@model Car

<div class="col-lg-4">
    <img class="img-thumbnail pic" src="@Model.img" alt="@Model.name" />
    <h2>Модель: @Model.name</h2>
    <p>@Model.shortDesc</p>
    <p>Цена: @Model.price.ToString("c")</p>
    <p><a class="btn btn-warning" asp-controller="ShopCart" asp-action="addToCart" asp-route-id="@Model.id">Добавить в корзину</a></p>
</div>

public class Startup
    {
        private IConfigurationRoot _confSting;
        public Startup(IWebHostEnvironment hostEnv)
        {
            _confSting = new ConfigurationBuilder().SetBasePath(hostEnv.ContentRootPath).AddJsonFile("dbsettings.json").Build();

        }
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)  
        {
            services.AddDbContext<AppDBContent>(options => options.UseSqlServer(_confSting.GetConnectionString("DefaultConnection")));
            services.AddControllersWithViews();
            services.AddTransient<IAllCars, CarRepository>();
            services.AddTransient<ICarsCategory, CategoryRepository>();
            services.AddTransient<IAllOrders, OrdersRepository>();

            services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
            services.AddScoped(sp => ShopCart.GetCart(sp));

            services.AddSession();
            services.AddMvc();
            services.AddMemoryCache();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            app.UseSession();
            app.UseRouting();     

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}");
                endpoints.MapControllerRoute(
                   name: "categoryFilter",
                   pattern: "Car/{action}/{category?}", defaults: new { Controller="Car", action="List"});
            });
            app.UseStaticFiles();

          

            
            using (var scope = app.ApplicationServices.CreateScope())
            {
                AppDBContent content = scope.ServiceProvider.GetRequiredService<AppDBContent>();
                DBObjects.Initial(content);
            }

        }
    }

Answer 1

after appDBContent.Order.Add(order); do SaveChanges, otherwise you will have order.id by default (0) and this is where the external index error pops up