EF Core DeleteBehavior.SetNull 造成循环问题

Question

我正在使用 EF Core 1.1.0，但级联行为存在严重问题。

我有一个名为 Land 的模型，如下所示：

public class Land
{
    public long Id { get; set; }
    public int HorizontalPosition { get; set; }
    public int VerticalPosition { get; set; }
    public bool IsPlaced { get; set; }

    // Relations
    public string UserId { get; set; }
    public virtual User User { get; set; }
    public long? BuildingId { get; set; }
    public virtual Building Building { get; set; }
}

和另一个名为 Building 的模型，如下所示：

public class Building
{
    public long Id { get; set; }
    public bool IsPermanent { get; set; }
    public int UpgradeCount { get; set; }

    // Relations
    public string UserId { get; set; }
    public virtual User User { get; set; }
    public int BuildingTypeId { get; set; }
    public virtual BuildingType BuildingType { get; set; }
    public virtual List<Land> Lands { get; set; }
}

在 dbcontext 中，我提到了一个关系：

...
modelBuilder.Entity<Land>()
            .HasOne(l => l.Building)
            .WithMany(b => b.Lands)
            .HasForeignKey(l => l.BuildingId)
            .OnDelete(Microsoft.EntityFrameworkCore.Metadata.DeleteBehavior.SetNull);
...

但是当我尝试更新数据库时，出现此错误：

System.Data.SqlClient.SqlException: Introducing FOREIGN KEY constraint 'FK_Lands_Buildings_BuildingId' on table 'Lands' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
Could not create constraint or index. See previous errors.
   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at System.Data.SqlClient.SqlCommand.RunExecuteNonQueryTds(String methodName, Boolean async, Int32 timeout, Boolean asyncWrite)
   at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(TaskCompletionSource`1 completion, Boolean sendToPipe, Int32 timeout, Boolean asyncWrite, String methodName)
   at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()
   at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.Execute(IRelationalConnection connection, String executeMethod, IReadOnlyDictionary`2 parameterValues, Boolean closeConnection)
   at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.ExecuteNonQuery(IRelationalConnection connection, IReadOnlyDictionary`2 parameterValues)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationCommandExecutor.ExecuteNonQuery(IEnumerable`1 migrationCommands, IRelationalConnection connection)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.Migrator.Migrate(String targetMigration)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.UpdateDatabase(String targetMigration, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.UpdateDatabase.<>c__DisplayClass0_1.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
ClientConnectionId:65b8e08e-2d17-46ac-b734-385f88dd07b1
Error Number:1785,State:0,Class:16
Introducing FOREIGN KEY constraint 'FK_Lands_Buildings_BuildingId' on table 'Lands' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
Could not create constraint or index. See previous errors.

生成的迁移是这样的：

...
migrationBuilder.AddForeignKey(
            name: "FK_Lands_Buildings_BuildingId",
            table: "Lands",
            column: "BuildingId",
            principalTable: "Buildings",
            principalColumn: "Id",
            onDelete: ReferentialAction.SetNull);
...

我已经告诉在删除时将外键设置为空，但它希望我不执行任何操作。尽管在 dbcontext 中没有称为“NoAction”的行为！

Answer 1

您看到的异常文本是由 SQL Server 生成的。

EF Core 没有任何等效的NO ACTION.

您唯一的方法是设置Restrict一个引用（或同时设置两个引用）。

Lands在删除父实体 ( ) 之前，您需要手动管理相关记录（例如更新或删除） User。或者，在呈现视图时检查子记录是否存在，并且不显示用于删除用户的链接/按钮。或者在删除、中止操作期间捕获异常并向用户显示相关消息。