问Entity Framework自动填充错误的实体
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Stack Overflow用户

提问于 2018-06-10 04:12:21

回答 1查看 67关注 0票数 1

我正在开发的音乐商店示例应用程序与ASP.NET的MVC，实体框架和WCF。

这是一个分层的应用程序，它有一个用于实体的公共层。

我已经写了一个种子方法，它工作得很好，但是当我试图向Cart添加一个Album时，另一个重复的Album也被添加到Album表中，同时还添加了重复的Artists。当我在填充Cart对象时添加Album = album语句时，就会发生这种情况。我添加了这条语句，因为当我创建视图时，它会为item.Album.Title条目生成null

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,
                        Album = album
                    };

                    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();
            }
}

模型类：

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; }
    }
}

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; }
    }
}

种子方法：

namespace MusicStore.Data
{
    internal class CommonDBInitializer : CreateDatabaseIfNotExists<MusicStoreEntities>
    {
        protected override void Seed(MusicStoreEntities context)
        {
            var genres1 = new List<Genre>
            {
                new Genre
                {
                    Name = "Rock",
                    Description = "Rock",
                    Albums = new List<Album>()
                },
                new Genre
                {
                    Name = "Jazz",
                    Description = "Jazz",
                    Albums = new List<Album>()
                },
                new Genre
                {
                    Name = "Metal",
                    Description = "Metal",
                    Albums = new List<Album>()
                },
                new Genre
                {
                    Name = "Alternative",
                    Description = "Alternative",
                    Albums = new List<Album>()
                },
                new Genre
                {
                    Name = "Disco",
                    Description = "Disco",
                    Albums = new List<Album>()
                },
                new Genre
                {
                    Name = "Blues",
                    Description = "Blues",
                    Albums = new List<Album>()
                },
                new Genre
                {
                    Name = "Latin",
                    Description = "Latin",
                    Albums = new List<Album>()
                },
                new Genre
                {
                    Name = "Reggae",
                    Description = "Reggae",
                    Albums = new List<Album>()
                },
                new Genre
                {
                    Name = "Pop",
                    Description = "Pop",
                    Albums = new List<Album>()
                },
                new Genre
                {
                    Name = "Classical",
                    Description = "Classical",
                    Albums = new List<Album>()
                },
            };
            genres1.ForEach(d => context.Genres.Add(d));
            context.SaveChanges();

            var artist1 = new List<Artist>
            {
                new Artist
                {
                    Name = "Aaron Copland & London Symphony Orchestra",

                },
                 new Artist
                {
                    Name = "Barry Wordsworth & BBC Concert Orchestra",

                },

            };

            artist1.ForEach(d => context.Artists.Add(d));
            context.SaveChanges();

            var album1 = new List<Album>
            {
                new Album { Title = "The Best Of Men At Work",  Price = 8.99M,  AlbumArtUrl = "/Content/Images/placeholder.gif",Genre = genres1.FirstOrDefault(d => d.GenreId == 1), Artist  = artist1.FirstOrDefault(d => d.ArtistId == 1) },
               new Album { Title = "A Copland Celebration, Vol. I",  Price = 8.99M, AlbumArtUrl = "/Content/Images/placeholder.gif",Genre = genres1.FirstOrDefault(d => d.GenreId == 2),Artist  = artist1.FirstOrDefault(d => d.ArtistId == 2) },

            };

            album1.ForEach(s => context.Albums.Add(s));

            context.SaveChanges();

        }
    }

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");
        }

    }
}

查看：

@model MusicStore.Web.ViewModels.ShoppingCartViewModel
@{
    ViewBag.Title = "Shopping Cart";
}
<script src="/Scripts/jquery-1.10.2.min.js" type="text/javascript"></script>

<h3>
    <em>Review</em> your cart:
</h3>
<p class="button">
    @Html.ActionLink("Checkout >>", "AddressAndPayment", "Checkout")
</p>
<div id="update-message">
</div>
<table>
    <tr>
        <th>
            Album Name
        </th>
        <th>
            Price (each)
        </th>
        <th>
            Quantity
        </th>
        <th></th>
    </tr>
    @foreach (var item in Model.CartItems)
    {
        <tr id="row-@item.RecordId">
            <td>
                @Html.ActionLink(item.Album.Title, "Details", "Store", new { id = item.AlbumId }, null)
            </td>
            <td>
                @item.Album.Price
            </td>
            <td id="item-count-@item.RecordId">
                @item.Count
            </td>
            <td>
                <a href="#" class="RemoveLink" data-id="@item.RecordId">Remove from cart</a>
            </td>
        </tr>
    }
    <tr>
        <td>
            Total
        </td>
        <td></td>
        <td></td>
        <td id="cart-total">
            @Model.CartTotal
        </td>
    </tr>
</table>

的问题是：为什么它会以如此不寻常的方式自动填充，以及如何修复它？

asp.net-mvc

entity-framework

wcf

回答 1

Stack Overflow用户

回答已采纳

发布于 2018-06-10 12:46:29

复制是因为在AddToCart方法中实例化了一个新的上下文MusicStoreEntities。该上下文不知道您通过album参数传递的Album实体的任何信息。

所以当你写这段代码的时候：

cartItem = new Cart
{
    // [...]
    Album = album
};

名为db.Carts.Add(cartItem);，cartItem将处于Added状态。EF还将检查cartItem对象图，并发现album未被新上下文跟踪(请记住，此实例来自您的方法的参数)，然后默认情况下，它也会将该实体置于Added状态。EF将对album对象图执行相同的操作，您也将在Added状态上获得该专辑的艺术家。当调用db.SaveChanges();时，每个具有Added状态的实体都会被认为是在数据库中生成一个新的插入，所以这就是为什么要进行复制。

要解决这个问题，您可以调用db.Attach(album)，让context来处理实体，如下所示：

using (MusicStoreEntities db = new MusicStoreEntities())
{
     db.Albums.Attach(album);

    // [...]
}

或者不使用album实例，您可以只设置外键属性AlbumId，而不使用Album属性赋值。因此，在创建新购物车项目时，您将拥有以下内容：

cartItem = new Cart
{
    AlbumId = album.AlbumId,
    CartId = ShoppingCartID,
    Count = 1,
    DateCreated = DateTime.Now
};

票数 1

页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持

原文链接：

https://stackoverflow.com/questions/50778136

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