inheritance - DbSet Add 方法不支持继承？

Question

我有以下 POCO 课程：

Public Class CompetitionSubscription
    Public Property ID As Long
    Public Overridable Property CompetitionID As Long
    Public Overridable Property CompetitionPlayerID As Long
    Public Property CompetitionStrength As Short?
    Public Property StrengthWithinSection As Short?
End Class

<NotMapped()>
Public Class FullCompetitionSubscription
    Inherits CompetitionSubscription

    Public Property Competition As Competition
    Public Property CompetitionPlayer As CompetitionPlayer

End Class

只有一个表的 DbContext 是这样的：

Public Class CompetitionSubscriptionsDB
    Inherits DbContext

    Public Property CompetitionSubscriptions As DbSet(Of CompetitionSubscription)

End Class

我写了一个这样的单元测试：

<TestMethod()>
Public Sub AddSubscriptionToDatabase()

     Using db = New CompetitionSubscriptionsDB
         Dim subscription = New FullCompetitionSubscription With
                          {.CompetitionID = 1,
                           .CompetitionPlayerID = 1,
                           .CompetitionStrength = 2,
                           .StrengthWithinSection = 12}
         db.CompetitionSubscriptions.Add(DirectCast(subscription, CompetitionSubscription))
         db.SaveChanges()
     End Using

 End Sub

这失败并出现以下错误（从荷兰语翻译，因此可能不是确切的消息）： No Metadata found for EntityType CompetitionSubscriptionsLib.FullCompetitionSubscription。

由于FullCompetitionSubscription该类装饰有<NotMapped>属性，我可以理解错误消息，但重点DirectCast是“出售”FullCompetitionSubscription作为基础CompetitionSubscription的DbContext（如果我将类型更改为subscription测试CompetitionSubscription通过）。

我哪里错了？