多对多删除/删除失败 w/“ObjectStateManager 中已存在具有相同键的对象”

Question

总结：使用 .attachto 的多个多对多关系添加工作正常，但相同的代码，除了是 .remove，而不是 .add，会导致可怕：

ObjectStateManager 中已存在具有相同键的对象。 ObjectStateManager 无法跟踪具有相同键的多个对象。

所以这里是代码。根据表单字段，一个隐藏（与之前的教练值），一个不隐藏，我循环浏览一个教练列表以查看已选择或取消选择哪些，然后添加或删除客户端和之间的多对多关系教练：

Protected Sub dsClient_Updated(sender As Object, e As System.Web.UI.WebControls.EntityDataSourceChangedEventArgs) Handles dsClient.Updated
    Dim Client As Client = e.Entity
    Dim Trainers As IEnumerable(Of aspnet_Users), CurrentTrainer As String, PrevTrainer As String

    ' Select list of Trainers
    Trainers = From S In FitnessDB.aspnet_Users

    For Each Trainer As aspnet_Users In Trainers
        CurrentTrainer = Request.Form(Trainer.UserName)
        PrevTrainer = Request.Form("Prev_" & Trainer.UserName)
        ' Check if Trainer has been deleted
        If CurrentTrainer = "" And PrevTrainer <> "" Then
            ' Trainer previously existed, delete
            FitnessDB.Detach(Trainer)
            e.Context.AttachTo ("aspnet_Users", Trainer)
            Client.aspnet_Users.Remove (Trainer)
        ElseIf CurrentTrainer <> "" And PrevTrainer = "" Then
            ' Doesn't exist, add
            FitnessDB.Detach(Trainer)
            e.Context.AttachTo ("aspnet_Users", Trainer)
            Client.aspnet_Users.Add (Trainer)
        End If
    Next
    e.Context.SaveChanges()
End Sub

我可以选择多个教练来添加就好了。它循环：

FitnessDB.Detach(Trainer)
e.Context.AttachTo ("aspnet_Users", Trainer)
Client.aspnet_Users.Add (Trainer)

没问题，添加新的关系就好了。如果我只删除一个教练，它工作正常。但是，如果我尝试使用以下代码删除多个培训师

FitnessDB.Detach(Trainer)
e.Context.AttachTo ("aspnet_Users", Trainer) <-- Error: An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.
Client.aspnet_Users.Remove (Trainer)

然后我在第二次删除尝试时收到错误。

那么让我困惑的是：为什么我在.add'ing时可以成功.attachto多个“trainer”，但在.remove'ing时却不行？

任何建议将不胜感激。

Answer 1

在这一点上，我得出的初步结论是它是实体框架中的一个错误，原因有很多，我太累了，无法在这里详细说明。

不过，我想出了一个解决方法。我已经删除了上面检查培训师删除的代码部分，并将其替换为：

For Each Trainer As aspnet_Users In Client.aspnet_Users
    If Request.Form(Trainer.UserName) = "" Then DeleteTrainers.Add (Trainer)
Next
For Each Trainer As aspnet_Users In DeleteTrainers
    Client.aspnet_Users.Remove (Trainer)
Next

重写的整个子现在是：

Protected Sub dsClient_Updated(sender As Object, e As System.Web.UI.WebControls.EntityDataSourceChangedEventArgs) Handles dsClient.Updated
    Dim Client As Client = e.Entity
    Dim DeleteTrainers As New List(Of aspnet_Users), CurrentTrainer As String, PrevTrainer As String

    ' Check for trainers that have been removed
    For Each Trainer As aspnet_Users In Client.aspnet_Users
        If Request.Form(Trainer.UserName) = "" Then DeleteTrainers.Add (Trainer)
    Next
    For Each Trainer As aspnet_Users In DeleteTrainers
        Client.aspnet_Users.Remove (Trainer)
    Next

    ' Check for trainers to add
    For Each Trainer As aspnet_Users In FitnessDB.aspnet_Users
        CurrentTrainer = Request.Form(Trainer.UserName)
        PrevTrainer = Request.Form("Prev_" & Trainer.UserName)
        If CurrentTrainer <> "" And PrevTrainer = "" Then
            ' Doesn't exist, add
            FitnessDB.Detach(Trainer)
            e.Context.AttachTo ("aspnet_Users", Trainer)
            Client.aspnet_Users.Add (Trainer)
        End If
    Next
    e.Context.SaveChanges()
End

通过改用 Client.aspnet_Users，我能够留在相同的上下文 (e.context) 中，从而消除了必须从 FitnessDB 上下文分离并重新附加到 e.context 的问题，然后崩溃当我删除实体 Trainer 时退出。

通过首先循环遍历相关培训师的表格并将它们添加到列表中，然后我可以通过循环遍历列表来成功删除它们（我不能直接在表格循环中删除它们，因为这会出错带有“集合已修改；枚举操作可能无法执行。”的循环。

希望有一天这对某人有所帮助。