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Question

I have problem constructing my relationships.

I want every Post to has one User and many Comments . Every User to have many Comments and Posts (if I want anytime to filter comments and posts per User). And then every Comment to has one User and Post

The problem is when I seed the db

if (!ctx.Posts.Any()) {
    Tag tag = new Tag() {
        Name = "Test1"
    };

    User user = new User() {
        UserName = "TestUser",
        Email = "test1@test1.bg",
        Password = Extensions.PasswordHash("abv123456", config["AppSettings:PasswordSalt"]),
        Role = UserRoles.Member
    };

    ctx.Users.Add(user);

    Post post = new Post() {
        Title = "Test Post",
        Description = "Description post test",
        Likes = 5,
        User = user,
        CreateOn = DateTime.Now,
        LastEditOn = DateTime.Now
    };

    PostTag posttags = new PostTag() {
        Post = post,
        Tag = tag
    };

    ctx.PostTag.Add(posttags);
    ctx.SaveChanges();
};

And If I want to fetch all posts with the related User

public IActionResult GetAllPost() {
    var posts = _ctx.Posts
        .Include(u => u.User)
        .ToList();

    return Json(Newtonsoft.Json.JsonConvert.SerializeObject(posts));
}

I get Newtonsoft.Json.JsonSerializationException: Self referencing loop detected with type 'Blog.Data.Entities.Post'. Path '[0].User.Posts'.

While in my Configure method I did ignore it

services.AddMvc().AddJsonOptions(opts => {
    opts.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
});

OnModelCreating()

protected override void OnModelCreating(ModelBuilder modelBuilder) {
    // set unique email and username
    modelBuilder.Entity<User>()
        .HasIndex(u => new { u.Email, u.UserName })
        .IsUnique();

    // set unique tag name
    modelBuilder.Entity<Tag>()
        .HasIndex(t => t.Name)
        .IsUnique();

    modelBuilder.Entity<Post>()
        .HasOne(p => p.User)
        .WithMany(u => u.Posts);

    modelBuilder.Entity<User>()
        .HasMany(u => u.Comments)
        .WithOne(u => u.User);

    // post tag many-to-many rel
    modelBuilder.Entity<PostTag>()
        .HasKey(pt => new { pt.PostUID, pt.TagUID });

    modelBuilder.Entity<PostTag>()
        .HasOne(pt => pt.Post)
        .WithMany(p => p.PostTags)
        .HasForeignKey(pt => pt.PostUID);

    modelBuilder.Entity<PostTag>()
        .HasOne(pt => pt.Tag)
        .WithMany(p => p.PostTags)
        .HasForeignKey(pt => pt.TagUID);
} 

Answer 1

You are trying to JSONify a JSON string:

return Json(Newtonsoft.Json.JsonConvert.SerializeObject(posts));

You need to let the framework do it for you, instead:

return Json(posts);

Answer 2

The problem is you've configured your service, but you're not using the service. You're using a static reference to Newtonsoft Json converter. You either need to use the service provided by framework (it should use it automatically when returning a model from a controller), or apply the configuration to the Newtonsoft Json converter call that you're using.