异常有关外键

Question

下面是内部异常：

InnerException: System.Data.UpdateException 
Message=Entities in 'OrganizerDBEntities.Assignments' participate in 
the 'CourseId' relationship. 0 related 'Course' were found. 
1 'Course' is expected.` 

我有三个表（文件夹，作业，课程）。 Assignment表有一个名为AssignmentId的主键和一个名为CourseId的“允许空值”属性设置为true的外键。所以这个异常阻止了我调用_entities.SaveChanges（）;从而阻止我将数据添加到数据库。

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UPDATE：多亏了马克，这个问题解决了，但彼起：

InnerException: System.Data.UpdateException 
     Message=An error occurred while updating the entries. See the InnerException for details. 
     Source=System.Data.Entity 
     StackTrace: 
      at System.Data.Mapping.Update.Internal.UpdateTranslator.Update(IEntityStateManager stateManager, IEntityAdapter adapter) 
      at System.Data.EntityClient.EntityAdapter.Update(IEntityStateManager entityCache) 
      at System.Data.Objects.ObjectContext.SaveChanges(Boolean acceptChangesDuringSave) 
      at System.Data.Objects.ObjectContext.SaveChanges() 
      at AssignmentOrganizer.App_Data.AssignmentRepository.CreateAssignment(Assignment assignmentToCreate) in C:\Users\Mohit\Documents\Visual Studio 2010\Projects\AssignmentOrganizer\AssignmentOrganizer\App_Data\AssignmentRepository.cs:line 19 
      at AssignmentOrganizer.MainWindow..ctor() in C:\Users\Mohit\Documents\Visual Studio 2010\Projects\AssignmentOrganizer\AssignmentOrganizer\MainWindow.xaml.cs:line 54 
     InnerException: System.Data.EntityCommandCompilationException 
      Message=An error occurred while preparing the command definition. See the inner exception for details. 
      Source=System.Data.Entity 
      StackTrace: 
        at System.Data.Mapping.Update.Internal.UpdateTranslator.CreateCommand(DbModificationCommandTree commandTree) 
        at System.Data.Mapping.Update.Internal.DynamicUpdateCommand.CreateCommand(UpdateTranslator translator, Dictionary`2 identifierValues) 
        at System.Data.Mapping.Update.Internal.DynamicUpdateCommand.Execute(UpdateTranslator translator, EntityConnection connection, Dictionary`2 identifierValues, List`1 generatedValues) 
        at System.Data.Mapping.Update.Internal.UpdateTranslator.Update(IEntityStateManager stateManager, IEntityAdapter adapter) 
      InnerException: System.NotSupportedException 
        Message=Server-generated keys and server-generated values are not supported by SQL Server Compact. 
        Source=System.Data.SqlServerCe.Entity 
        StackTrace: 
         at System.Data.SqlServerCe.SqlGen.DmlSqlGenerator.GenerateReturningSql(StringBuilder commandText, DbModificationCommandTree tree, ExpressionTranslator translator, DbExpression returning) 
         at System.Data.SqlServerCe.SqlGen.DmlSqlGenerator.GenerateInsertSql(DbInsertCommandTree tree, List`1& parameters, Boolean isLocalProvider) 
         at System.Data.SqlServerCe.SqlGen.SqlGenerator.GenerateSql(DbCommandTree tree, List`1& parameters, CommandType& commandType, Boolean isLocalProvider) 
         at System.Data.SqlServerCe.SqlCeProviderServices.CreateCommand(DbProviderManifest providerManifest, DbCommandTree commandTree) 
         at System.Data.SqlServerCe.SqlCeProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree) 
         at System.Data.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree) 
         at System.Data.Common.DbProviderServices.CreateCommand(DbCommandTree commandTree) 
         at System.Data.Mapping.Update.Internal.UpdateTranslator.CreateCommand(DbModificationCommandTree commandTree) 
        InnerException: 

Answer 1

我会检查的第一件事是在你的EF概念模型的关联OrganizerDBEntities.Assignments。它是否允许0：n关系？从错误信息中，我可以相信它没有。您可以调整EF模型中的基数（最小和最大可能值）。

如果您选择EDMX设计器表面中两个实体之间的关联，则属性窗口将显示关联的两端。在每个“End”属性中，都有一个“Multiplicity”值，它定义了关联的基数：0:*,0..1:*,1:*等等。

如果你确实想要允许一个缺失的值，你的关联子结点的多样性需要是0..1（零或一） - 它是真的0..1（零还是一）？或者它是否设置为1？

Answer 2

在这里我的2美分。
我只是避免将外键值设置为NULL.So而不是设置CourseId作为分配表中的外键，将AssignmentId设置为课程表中的外键。