How to delete row with 1:1 relation to the same table? - entity-framework

Question

I using Entity Framework Core, and I have a 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; }
}
From this, EFC generates the following 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);
Some comments are send as a reply to another, but not all. When the original comment is deleted, the reply becomes a normal comment. So, following this tutorial, the configuration looks is this:
modelBuilder.Entity<BlogComment>()
.HasOne(p => p.ReplyTo)
.WithOne()
.HasForeignKey<BlogComment>(c => c.ReplyToId)
.IsRequired(false)
.OnDelete(DeleteBehavior.SetNull);
The delete method is pretty simple:
var comment = await context.BlogComment.Include(c => c.ReplyTo).SingleAsync(m => m.Id == id);
context.BlogComment.Remove(comment);
await context.SaveChangesAsync();
But I can't run it, I get an error:
System.Data.SqlClient.SqlException: The DELETE statement conflicted with the SAME TABLE REFERENCE constraint "FK_BlogComment_BlogComment_ReplyToId".
How can I fix this?

Answer 1

To wrap up the conversation in the comments:
First, the self reference is a 1:n association:
modelBuilder.Entity<BlogComment>()
.HasOne(p => p.ReplyTo)
.WithMany(c => c.Replies)
.HasForeignKey(c => c.ReplyToId)
.IsRequired(false)
.OnDelete(<we'll get to that>);
So, just for convenience, BlogComment now also has a property
public ICollection<BlogComment> Replies { get; set; }
However, I can't create the table using
.OnDelete(DeleteBehavior.SetNull);
It gives me
Introducing FOREIGN KEY constraint 'FK_BlogComments_BlogComments_ReplyToId' on table 'BlogComments' may cause cycles or multiple cascade paths.
This is a Sql Server restriction we just have to accept, no way to evade it. The only way to get the desired cascade behavior is
.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.
I.e.: the client executes SQL to nullify the foreign key values. The child records should be tracked though. To remove a BlogComment parent the delete action should look like:
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();
}
As you see, you don't have to set ReplyToId = null, that's something EF takes care of.

Answer 2

For me, I had to Include() the entities I needed to be "dealt with" when I deleted an entity. EF cant manage things it is not currently tracking.
var breedToDelete = context.Breed
.Include(x => x.Cats)
.Single(x => x.Id == testBreedId);
context.Breed.Remove(breedToDelete);
context.SaveChanges();

Answer 3

I could get it working by manually setting ReplyTo to null. I'm still looking for a better solution, or an explanation why is it needed. Isn't it what OnDelete(DeleteBehavior.SetNull) supposed to do?
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);