如何删除与 table 具有 1:1 关系的行？

Question

我使用 Entity Framework 核心，我有一个 table:

public class BlogComment
{
    public int Id { get; set; }
    public BlogPost Post { get; set; }
    [StringLength(100)]
    public string AuthorName { get; set; }
    [StringLength(254)]
    public string AuthorEmail { get; set; }
    public bool SendMailOnReply { get; set; }
    [StringLength(2000)]
    public string Content { get; set; }
    public DateTime CreatedTime { get; set; }
    public int? ReplyToId { get; set; }
    public BlogComment ReplyTo { get; set; }
}

据此，EFC 生成以下 table:

CREATE TABLE [dbo].[BlogComment] (
    [Id]              INT             IDENTITY (1, 1) NOT NULL,
    [AuthorEmail]     NVARCHAR (254)  NULL,
    [AuthorName]      NVARCHAR (100)  NULL,
    [Content]         NVARCHAR (2000) NULL,
    [CreatedTime]     DATETIME2 (7)   NOT NULL,
    [PostId]          INT             NULL,
    [ReplyToId]       INT             NULL,
    [SendMailOnReply] BIT             NOT NULL,
    CONSTRAINT [PK_BlogComment] PRIMARY KEY CLUSTERED ([Id] ASC),
    CONSTRAINT [FK_BlogComment_BlogPost_PostId] FOREIGN KEY ([PostId]) REFERENCES [dbo].[BlogPost] ([Id]),
    CONSTRAINT [FK_BlogComment_BlogComment_ReplyToId] FOREIGN KEY ([ReplyToId]) REFERENCES [dbo].[BlogComment] ([Id])
);
GO
CREATE NONCLUSTERED INDEX [IX_BlogComment_PostId]
    ON [dbo].[BlogComment]([PostId] ASC);
GO
CREATE UNIQUE NONCLUSTERED INDEX [IX_BlogComment_ReplyToId]
    ON [dbo].[BlogComment]([ReplyToId] ASC) WHERE ([ReplyToId] IS NOT NULL);

有些评论是作为对另一条评论的回复发送的，但不是全部。删除原评论后，回复就变成普通评论了。所以，按照 this tutorial，配置看起来是这样的：

modelBuilder.Entity<BlogComment>()
      .HasOne(p => p.ReplyTo)
      .WithOne()
      .HasForeignKey<BlogComment>(c => c.ReplyToId)
      .IsRequired(false)
      .OnDelete(DeleteBehavior.SetNull);

删除方法很简单：

var comment = await context.BlogComment.Include(c => c.ReplyTo).SingleAsync(m => m.Id == id);
context.BlogComment.Remove(comment);
await context.SaveChangesAsync();

但是我不能运行，我得到一个错误：

System.Data.SqlClient.SqlException: The DELETE statement conflicted with the SAME TABLE REFERENCE constraint "FK_BlogComment_BlogComment_ReplyToId".

我该如何解决这个问题？

Answer 1

我可以通过手动将 ReplyTo 设置为空来让它工作。我仍在寻找更好的解决方案，或者解释为什么需要它。这不是OnDelete(DeleteBehavior.SetNull)应该做的吗？

var comment = await context.BlogComment.Include(c => c.ReplyTo).SingleAsync(m => m.Id == id);
var reply = await context.BlogComment.SingleOrDefaultAsync(m => m.ReplyToId == id);
if (reply != null)
{
    reply.ReplyTo = null;
    reply.ReplyToId = null;
    context.Entry(reply).State = EntityState.Modified;
}
context.BlogComment.Remove(comment);

Answer 2

在评论中结束对话：

首先，自引用是一个1:n关联：

modelBuilder.Entity<BlogComment>()
      .HasOne(p => p.ReplyTo)
      .WithMany(c => c.Replies)
      .HasForeignKey(c => c.ReplyToId)
      .IsRequired(false)
      .OnDelete(<we'll get to that>);

所以，为了方便起见，BlogComment 现在也有一个属性

public ICollection<BlogComment> Replies { get; set; }

但是，我无法使用

创建 table

.OnDelete(DeleteBehavior.SetNull);

它给了我

Introducing FOREIGN KEY constraint 'FK_BlogComments_BlogComments_ReplyToId' on table 'BlogComments' may cause cycles or multiple cascade paths.

这是Sql服务器限制，我们只能接受，无法规避。获得所需级联行为的唯一方法是

.OnDelete(DeleteBehavior.ClientSetNull);

Which is:

For entities being tracked by the DbContext, the values of foreign key properties in dependent entities are set to null. This helps keep the graph of entities in a consistent state while they are being tracked, such that a fully consistent graph can then be written to the database. (...) This is the default for optional relationships.

即：客户端执行SQL来使外键值无效。但是应该跟踪子记录。要删除 BlogComment 父项，删除操作应如下所示：

using (var db = new MyContext(connectionString))
{
    var c1 = db.BlogComments
        .Include(c => c.Replies) // Children should be included
        .SingleOrDefault(c => c.Id == 1);
    db.BlogComments.Remove(c1);
    db.SaveChanges();
}

如您所见，您不必设置 ReplyToId = null，这是 EF 会处理的事情。

Answer 3

对我来说，当我删除一个实体时，我必须 Include() 我需要成为 "dealt with" 的实体。 EF 无法管理它当前未跟踪的内容。

var breedToDelete = context.Breed
    .Include(x => x.Cats)
    .Single(x => x.Id == testBreedId);

context.Breed.Remove(breedToDelete);
context.SaveChanges();

如何删除与 table 具有 1:1 关系的行？

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