努力使用实体框架将数据保存到ASP.NET MVC中的数据库

我刚刚开始学习C＃和ASP.NET MVC框架，并且正在尝试构建购物车功能。这是我认为的（添加到购物车方法）应该起作用，但不起作用。它将一个项目添加到购物车，但是每当我添加另一个项目时，上一个项目就消失了。有人知道为什么我的数据库保存不起作用吗？我是否以正确的方式使用上下文？

namespace e_commerse.Controllers
{
    public class ShoppingcartController : Controller
    {
            private ProductDBContext db = new ProductDBContext();

            [Authorize]
            public ActionResult addToCart(int ID)
            {
                Product product = db.Products.Find(ID);

                if (product == null)
                {
                    return HttpNotFound();
                }

                var userId =  User.Identity.GetUserId();

                IEnumerable<ShoppingCart> shoppingcarts = 
                    from cart in db.Shoppingcarts
                    where cart.userID == userId
                    select cart;

                if (!shoppingcarts.Any())
                {
                    var newShoppingcart = new ShoppingCart();
                    newShoppingcart.products = new List<Product>();
                    newShoppingcart.products.Add(product);
                    newShoppingcart.userID = userId;
                    db.Shoppingcarts.Add(newShoppingcart);
                    db.SaveChanges();
                    return View("Index", newShoppingcart);
                }

                var shoppingcart = shoppingcarts.First();

                if (shoppingcart.products == null)
                {
                    shoppingcart.products = new List<Product>();
                }

                shoppingcart.products.Add(product);
                shoppingcart.userID = userId;
                db.SaveChanges();

                return View("Index", shoppingcart);
            }

            protected override void Dispose(bool disposing)
            {
                if (disposing)
                {
                    db.Dispose();
                }
                base.Dispose(disposing);
            }
        }

        public class ProductDBContext : DbContext
        {
            public DbSet<Product> Products { get; set; }
            public DbSet<ShoppingCart> Shoppingcarts { get; set; }
        }

        public class ShoppingCart
        {
            public int ID { get; set; }
            public String userID { get; set; }
            public List<Product> products { get; set; }
        }

        public class Product
        {
            public int ID { get; set; }
            public String title { get; set; }
            public String description { get; set; }
            public decimal price { get; set; }
            public String imagepath { get; set; }
            public Type type { get; set; }
        }
    }

非常感谢您的帮助，并请原谅我的noob代码！