为什么我有一个错误：数据库操作预计会影响 1 行，但实际上影响了 0 行？

Question

我有一个错误： Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException: '数据库操作预计会影响 1 行，但实际上影响了 0 行；自加载实体后，数据可能已被修改或删除。

当我尝试使用实体框架向数据库中的帖子添加评论并保存更改时。我不知道为什么。我做错了什么？

这里我有错误：

[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    private readonly ApplicationDbContext _applicationDbContext;
    public ValuesController(ApplicationDbContext applicationDbContext)
    {
        _applicationDbContext = applicationDbContext;
    }

    [HttpGet]
    public IActionResult Get()
    {
        var post = _applicationDbContext.Posts.First();
        post.AddComment(Guid.NewGuid(), "aaa", "bbb");
        _applicationDbContext.SaveChanges(); // here error

        return Ok();
    }
}

实体框架配置 - 这里可能有问题？：

public class PostConfiguration : IEntityTypeConfiguration<Post>
{
    public void Configure(EntityTypeBuilder<Post> builder)
    {
        builder.ToTable("Posts");
        builder.HasKey(x => x.PostId);

        builder.HasMany(x => x.Comments).WithOne().HasForeignKey("PostId");
    }
}

public class CommentConfiguration : IEntityTypeConfiguration<Comment>
{
    public void Configure(EntityTypeBuilder<Comment> builder)
    {
        builder.ToTable("Comments");
        builder.HasKey(x => x.CommentId);

        builder.Property(x => x.CommentStatus)
            .HasColumnName("CommentStatusId")
            .HasConversion(new EnumToNumberConverter<CommentStatus, int>());
    }
}

public class ApplicationDbContext : DbContext
{
    public DbSet<Post> Posts { get; set; }
    public DbSet<Comment> Comments { get; set; }


    public ApplicationDbContext(
        DbContextOptions options) : base(options)
    {
    }


    public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken = new CancellationToken())
    {
        var result = await base.SaveChangesAsync(cancellationToken);

        return result;
    }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.ApplyConfigurationsFromAssembly(Assembly.GetExecutingAssembly());

        base.OnModelCreating(builder);
    }
}

这是我的帖子类：

public class Post
{
    public Guid PostId { get; private set; }

    public string Title { get; private set; }

    public string Content { get; private set; }               
    
    public List<Comment> Comments { get; private set; } = new List<Comment>();


    public Post(Guid postId, string title, string content)
    {
        PostId = postId;
        Title = title;
        Content = content;
    }


    public void AddComment(Guid commentId, string author, string content)
    {
        var comment = new Comment(commentId, author, content);
        Comments.Add(comment);
    }
}

这是我的评论类：

public class Comment
{
    public Guid CommentId { get; private set; }

    public string Author { get; private set; }

    public string Content { get; private set; }

    public CommentStatus CommentStatus { get; private set; }


    public Comment(Guid commentId, string author, string content)
    {
        CommentId = commentId;
        Author = author;
        Content = content;
        CommentStatus = CommentStatus.New;
    }
}   

这是我的 CommentStatus 类：

public enum CommentStatus
{
    New = 1,
    Accepted = 2,
    Rejected = 3
}

在 Program.cs 我有：

var connectionString = builder.Configuration.GetConnectionString("DefaultConnection");
builder.Services.AddDbContext<ApplicationDbContext>(options => options.UseSqlServer(connectionString));

Answer 1

好的，我知道了，我要补充一下：

builder.Property(x => x.Postd).ValueGeneratedNever(); 
builder.Property(x => x.CommentId).ValueGeneratedNever();