Question

我正在使用ASP.NET MVC，实体框架和WCF开发音乐商店示例应用程序。

这是一个分层应用程序，它具有实体的公共层。

在AddToCart Action方法中，Album对象填充正常，但在wcf加载购物车对象后保存购物车后，关联的Album对象为空，可能是某些序列化问题（我不知道），在视图@foreach (var item in Model.CartItems)中，item.Album.Title变为空

这是我的代码：

public static void AddToCart(Album album, string ShoppingCartID)
{
            using (MusicStoreEntities db = new MusicStoreEntities())
            {
                // Get the matching cart and album instances
                var cartItem = db.Carts.SingleOrDefault(
                    c => c.CartId == ShoppingCartID
                         && c.AlbumId == album.AlbumId);

                if (cartItem == null)
                {
                    // Create a new cart item if no cart item exists
                    cartItem = new Cart
                    {
                        AlbumId = album.AlbumId,
                        CartId = ShoppingCartID,
                        Count = 1,
                        DateCreated = DateTime.Now                           
                    };

                    db.Carts.Add(cartItem);
                }
                else
                {
                    // If the item does exist in the cart, then add one to the quantity
                    cartItem.Count++;
                }

                // Save changes
                db.SaveChanges();
            }
}

public static List<Cart> GetCartItems(string ShoppingCartID)
        {
            using (MusicStoreEntities db = new MusicStoreEntities())
            {
               return db.Carts.Where(cart => cart.CartId == ShoppingCartID).ToList();

            }
        }

控制器

namespace MusicStore.Web.Controllers
{
    public class ShoppingCartController : Controller
    {
        MusicShoppingCartMgr.Cart serviceref1 = new MusicShoppingCartMgr.Cart();
        MusicShoppingCartMgr.iShoppingCart servicemethodref1 = new iShoppingCartClient();
        //
        // GET: /ShoppingCart/
        public ActionResult Index()
        {
            var cart = ShoppingCart.GetCart(this.HttpContext);

            // Set up our ViewModel
            var viewModel = new ShoppingCartViewModel
            {
                CartItems = cart.GetCartItems(cart.ShoppingCartId),
                CartTotal = cart.GetTotal(cart.ShoppingCartId)
            };

            // Return the view
            return View(viewModel);
        }
        //
        // GET: /Store/AddToCart/5

        public ActionResult AddToCart(int id)
        {
            var addedAlbum = servicemethodref1.GetAlbum(id);

            // Add it to the shopping cart
            var cart = ShoppingCart.GetCart(this.HttpContext);

            cart.AddToCart(addedAlbum, cart.ShoppingCartId);

            // Go back to the main store page for more shopping
            return RedirectToAction("Index");
        }


    }
}

模型类

namespace MusicStore.Core
{
    [Serializable]
    [DataContract]
    public class Cart
    {
        [Key]
        [DataMember]
        public int RecordId { get; set; }
        [DataMember]
        public string CartId { get; set; }
        [DataMember]
        public int AlbumId { get; set; }
        [DataMember]
        public int Count { get; set; }
        [DataMember]
        public System.DateTime DateCreated { get; set; }
        [DataMember]
        public virtual Album Album { get; set; }
    }
}

namespace MusicStore.Core
{
    [Serializable]
    [DataContract]
    //[Bind(Exclude = "AlbumId")]
    public class Album
    {
        [DataMember]
        [ScaffoldColumn(false)]
        public int AlbumId { get; set; }

        [DataMember]
        [DisplayName("Genre")]
        public int GenreId { get; set; }

        [DataMember]
        [DisplayName("Artist")]
        public int ArtistId { get; set; }

         [DataMember]
        [Required(ErrorMessage = "An Album Title is required")]
        [StringLength(160)]
        public string Title { get; set; }

         [DataMember]
        [Required(ErrorMessage = "Price is required")]
        [Range(0.01, 100.00,
            ErrorMessage = "Price must be between 0.01 and 100.00")]
        public decimal Price { get; set; }

         [DataMember]
        [DisplayName("Album Art URL")]
        [StringLength(1024)]
        public string AlbumArtUrl { get; set; }

           [DataMember]
         public virtual Genre Genre { get; set; }
           [DataMember]
        public virtual Artist Artist { get; set; }
         public virtual List<OrderDetail> OrderDetails { get; set; }
    }
}

数据库上下文

namespace MusicStore.Data
{
    public class MusicStoreEntities : DbContext
    {
        public MusicStoreEntities()
            : base("MusicStoreEntities")
        {
            var ensureDLLIsCopied = System.Data.Entity.SqlServer.SqlProviderServices.Instance;

            this.Configuration.ProxyCreationEnabled = false;

            Database.SetInitializer<MusicStoreEntities>(new CreateDatabaseIfNotExists<MusicStoreEntities>());
            Database.SetInitializer(new CommonDBInitializer());
        }

        public DbSet<Album> Albums { get; set; }
        public DbSet<Genre> Genres { get; set; }
        public DbSet<Artist> Artists { get; set; }
        public DbSet<Cart> Carts { get; set; }
        public DbSet<Order> Orders { get; set; }
        public DbSet<OrderDetail> OrderDetails { get; set; }
    }
}

问题是：为什么Album对象未在Cart对象中加载，以及如何修复它？

Answer 1

实体框架默认不加载相关对象，以帮助防止将一对多或多对多关系加载到内存中的潜在性能问题。

查看您发布的代码，在.Include("Album")中取出购物车商品时，您可能会添加GetCartItems。然后它会变成

public static List<Cart> GetCartItems(string ShoppingCartID)
{
    using (MusicStoreEntities db = new MusicStoreEntities())
    {
        return db.Carts.Include("Album").Where(cart => cart.CartId == ShoppingCartID).ToList();
    }
}

有关加载相关实体的其他选项，请参阅the Entity Framework docs

实体框架不加载相关对象

1 个答案: