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我是 Entity Framework 的新手，我已经为导航对象的问题苦苦挣扎了一天多。我怀疑我正在尝试以一种不合适的方式与 EF 合作。我正在使用框架 4 和 EF v4。

我创建了一个示例模型来大致演示我遇到的问题。我的实际系统是一个 ASP.Net 应用程序。我们有一个页面，允许用户创建“DisplayBoards”并向其中添加一个或多个预先存在的“Bays”。在保存之前，用户有机会调整显示托架的顺序。这记录在链接记录上。

因为链接记录显示在列表中（按顺序），所以在选择每个托架时，我一直在创建一个新的链接对象，添加托架，然后使用链接对象列表绑定到网格。这工作正常。

尝试将链接对象添加到新的 DisplayBoard 并保存所有内容时，最终回发时会出现问题。

在新链接对象上调用“AddObject”时会引发异常：“EntityKey 属性只能在属性的当前值为 null 时设置。”

这是基本的表结构：

DisplayBoard
DisplayBoardID（PK，身份）
DisplayBoardName

BoardBayLink
LinkID (PK, identity)
DisplayBoardID (FK)
BayID (FK)
DisplayOrder (int)

Bay
BayID (PK, 身份)
BayName

这是一些示例代码。这是一个使用三个上下文的控制台应用程序，但这是为了模拟 ASP.Net 应用程序上的回发。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace EF_Test
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Entity Framework Issue");
            Console.WriteLine("Setting FK outside of context");
            Console.WriteLine();

            DisplayBoard newBoard = null;
            Bay existingBayOne = null;
            Bay existingBayTwo = null;
            List<BoardBayLink> newLinksList = new List<BoardBayLink>();

            /* In this example there are two contexts.
             * This is to simulate an ASP.Net scenario...
             *      First, a new DisplayBoard is created at UI level.
             *      A Bay is searched for and selected via callback or postback.
             *      A new BoardBayLink is created at UI level and the Bay record is added.
             *      Once user is happy, final postback to save.
             *      
             * This is why there are two contexts here in a console app.
             */

            // New display board created
            newBoard = new DisplayBoard() { DisplayBoardName = "My new display board" };

            // User searches for and selects a Bay (via postback)
            using (EntityFrameworkTestEntities context = new EntityFrameworkTestEntities())
            {
                existingBayOne = context.Bays.Where(bay => bay.BayID == 1).FirstOrDefault();
            }
            // Now that the user has selected a bay, create a new BoardBayLink object and add the Bay to it.
            newLinksList.Add(new BoardBayLink() { Bay = existingBayOne, DisplayOrder = 1 });


            // User searches for and selects another Bay (via postback)
            using (EntityFrameworkTestEntities context2 = new EntityFrameworkTestEntities())
            {
                existingBayTwo = context2.Bays.Where(bay => bay.BayID == 2).FirstOrDefault();
            }
            // Now that the user has selected a second bay, create a new BoardBayLink object and add the Bay to it.
            newLinksList.Add(new BoardBayLink() { Bay = existingBayTwo, DisplayOrder = 2 });


            // Show the new board with it's newly added Bays in the UI
            // User has not yet clicked save.
            Console.Write("Display board: ");
            Console.WriteLine(newBoard.DisplayBoardName);

            Console.WriteLine("Bays: ");
            foreach (BoardBayLink link in newLinksList.OrderBy(link => link.DisplayOrder))
            {
                Console.WriteLine(link.Bay.BayName);
            }


            Console.WriteLine("Press any key to continue with save....");
            Console.ReadKey();

            // Now use a third context (because we're pretending this is a postback)
            using (EntityFrameworkTestEntities context3 = new EntityFrameworkTestEntities())
            {
                context3.DisplayBoards.AddObject(newBoard); // Add the new board to the context

                foreach (BoardBayLink link in newLinksList)
                {
                    context3.BoardBayLinks.AddObject(link);
                    newBoard.BoardBayLinks.Add(link);
                }

                context3.SaveChanges();
            }

            Console.WriteLine("Press any key to exit...");
            Console.ReadKey();
        }
    }
}

如何将托架添加到链接记录并避免我看到的错误消息？

谢谢

4

1 回答 1

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您无法再次添加链接，您已经在上下文之外创建了关系

newLinksList.Add(new BoardBayLink() { Bay = ... };

尝试再次添加它newBoard.BoardBayLinks.Add(link)会导致错误。

要添加新链接，您只需在第三个上下文中执行以下操作：

using (EntityFrameworkTestEntities context3 = new EntityFrameworkTestEntities())
{
    context3.DisplayBoards.Add(newBoard);
    context3.SaveChanges();
}

调用该Add()方法将标记EntityState图中所有实例的Added，依次为每个实例创建一个新的数据库记录。

于 2013-05-01T07:23:12.180 回答