Entity Framework 4.1 - 插入先前删除的数据时，多对多关系数据中出现错误

Question

场景：

我有3个表，形成多对多关系：

Agent (AgentID (PK), AgentName)
AgentChannel (AgentID (PK), ChannelID (PK))
Channel (ChannelID (PK), ChannelName)

我不想在Agent和Channel表中添加/删除任何记录，而只想修改AgentChannel表。

我正在使用 EF 4.1、POCO 和自我跟踪实体。所有处理数据库的代码都包含在 WCF 层中，我将它们称为服务器端代码，并且我只能控制客户端代码中记录的添加/删除。

例如： 我有一个代理记录，有多条通道记录。我可以通过这种方式将现有通道链接到代理：

var channel = new Channel { ChannelID = 1 };
channel.MarkAsUnchanged();
agent.Channels.Add(channel);
// This will add new entry to AgentChannel table, but no change on Agent and Channel tables

我还可以通过这种方式删除通道到代理的链接：

var tempChannels = agent.Channels.ToList();
var channelToDelete = tempChannels.FirstOrDefault(c => c.ChannelID == 3);
agent.Channels.Remove(channelToDelete);
// This will remove the entry in AgentChannel, but no change on Agent and Channel tables.

我的问题：
如果我删除了一个通道，然后将一个与之前删除的通道具有相同 ChannelID 的新通道添加回 agent.Channels 集合中，我将收到此错误：

AcceptChanges 无法继续，因为该对象的键值与 ObjectStateManager 中的另一个对象冲突。

演示示例代码：

        //
        // I not able to call ObjectContext here because it was reside in server side.
        //

        //
        // This is client side code
        //
        var tempChannels = agent.Channels.ToList();
        var channelToDelete = tempChannels.FirstOrDefault(c => c.ChannelID == 3);

        // remove channel 3
        agent.Channels.Remove(channelToDelete);

        var channel = new Channel { ChannelID = 3 };
        channel.MarkAsUnchanged();

        // Add back channel 3
        agent.Channels.Add(channel);

        //
        // This is server side code (WCF layer)
        //
        try
        {
            using (var ctx = new testEntities())
            {
                ctx.ApplyChanges("Agents", agent); // <-- ERROR happen here
                ctx.SaveChanges();
            }
        }
        catch (Exception)
        {
            throw;
        }

然后我收到异常消息：

    System.InvalidOperationException was unhandled
  Message=AcceptChanges cannot continue because the object's key values conflict with another object in the ObjectStateManager. Make sure that the key values are unique before calling AcceptChanges.
  Source=System.Data.Entity
  StackTrace:
       at System.Data.Objects.ObjectStateManager.FixupKey(EntityEntry entry)
       at System.Data.Objects.EntityEntry.AcceptChanges()
       at System.Data.Objects.EntityEntry.ChangeObjectState(EntityState requestedState)
       at System.Data.Objects.ObjectStateManager.ChangeObjectState(Object entity, EntityState entityState)
       at TestManyToMany.SelfTrackingEntitiesContextExtensions.ChangeEntityStateBasedOnObjectState(ObjectContext context, IObjectWithChangeTracker entity) in F:\MyFile\Development\Temp\TestManyToMany\TestManyToMany\Model1.Context.Extensions.cs:line 728
       at TestManyToMany.SelfTrackingEntitiesContextExtensions.HandleEntity(ObjectContext context, EntityIndex entityIndex, RelationshipSet allRelationships, IObjectWithChangeTracker entity) in F:\MyFile\Development\Temp\TestManyToMany\TestManyToMany\Model1.Context.Extensions.cs:line 596
       at TestManyToMany.SelfTrackingEntitiesContextExtensions.ApplyChanges[TEntity](ObjectContext context, String entitySetName, TEntity entity) in F:\MyFile\Development\Temp\TestManyToMany\TestManyToMany\Model1.Context.Extensions.cs:line 84
       at TestManyToMany.Program.Main(String[] args) in F:\MyFile\Development\Temp\TestManyToMany\TestManyToMany\Program.cs:line 53
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

我希望我说清楚了。这是我现在面临的唯一问题，我已经没有主意了。

非常感谢。

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更新-答案

根据Ladislav Mrnka的评论完成测试后我得到了答案。

基本上，您需要添加回之前删除的同一实体，而不是其他实体，因为这会给您带来上面相同的错误。

这是客户端的示例代码，服务器端无需更改：

        int channelId = 1;

        var tempChannels = agent.Channels.ToList();
        var channelToDelete = tempChannels.FirstOrDefault(c => c.ChannelID == channelId);

        // remove channel 1
        agent.Channels.Remove(channelToDelete);


        //var channel = _allChannels.First(c => c.ChannelID == channelId);
        //agent.Channels.Add(channel);           <-- This line will give you ERROR because the channel entity is from _allChannels, but not from agent.Channels

        // Add back channel 1
        agent.Channels.Add(channelToDelete); //  <-- This is CORRECT

Answer 1

那是不允许的操作。如果您删除了频道并想再次添加它，则不得创建新实例。您必须使用旧的并且仅更改其状态。主要规则是每个具有唯一键的实体只能声明一次。您的代码会导致两个实例具有相同的 key = 错误。