Why do I have an error: The database operation was expected to affect 1 row(s), but actually affected 0 row(s)?

Question

I have an error: Microsoft.EntityFrameworkCore.DbUpdateConcurrencyException: 'The database operation was expected to affect 1 row(s), but actually affected 0 row(s); data may have been modified or deleted since entities were loaded.

when I am trying to add a comment to a post in database using Entity Framework and save changes. I don't know why. What am I doing wrong?

Here I have the error:

[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();
    }
}

Entity Framework configuration - probably something is wrong here?:

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);
    }
}

Here is my Post class:

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);
    }
}

Here is my Comment class:

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;
    }
}   

Here is my CommentStatus class:

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

In Program.cs I have:

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

Answer 1

OK, I know, I have to add:

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