using System.Collections.Generic;

namespace Web.Tests
{
    public class Post
    {
        public int Id { get; set; }
        public string Title { get; set; }
        public string Content { get; set; }

        public IList<PostTag> PostTags { get; } = new List<PostTag>(); // Collection navigation
    }

    public class Tag
    {
        public int Id { get; set; }
        public string Text { get; set; }

        public IList<PostTag> PostTags { get; } = new List<PostTag>(); // Collection navigation
    }

    public class PostTag
    {
        public int PostId { get; set; } // First part of composite PK; FK to Post
        public int TagId { get; set; } // Second part of composite PK; FK to Tag

        public Post Post { get; set; } // Reference navigation
        public Tag Tag { get; set; } // Reference navigation
    }
}

using Microsoft.EntityFrameworkCore;

namespace Web.Tests
{
    public class BlogContext : DbContext
    {
        public BlogContext(DbContextOptions<BlogContext> options) : base(options) { }

        public DbSet<Post> Posts { get; set; }
        public DbSet<Tag> Tags { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Post>()
                .HasData(
                    new Post { Id = 1, Title = "How to use vs?", Content = "blababa" }
                );

            modelBuilder.Entity<Tag>()
                .HasData(
                    new Tag { Id = 1, Text = ".NET" },
                    new Tag { Id = 2, Text = "C#" }

                );
            modelBuilder.Entity<PostTag>()
                .HasKey(x => new { x.PostId, x.TagId });

            modelBuilder.Entity<PostTag>()
                .HasOne(x => x.Post)
                .WithMany(x => x.PostTags);

            modelBuilder.Entity<PostTag>()
                .HasOne(x => x.Tag)
                .WithMany(x => x.PostTags);

            modelBuilder.Entity<PostTag>()
                .HasData(
                    new PostTag { PostId = 1, TagId = 1 },
                    new PostTag { PostId = 1, TagId = 2 }
                );

            base.OnModelCreating(modelBuilder);
        }
    }
}

        [Fact]
        public void Test1()
        {
            var ctx = CreateContext();
            var post = ctx.Posts.Include(x => x.PostTags).SingleOrDefault(x => x.Id == 1);
            if (post != null)
            {
                post.PostTags.RemoveAt(1);
            }

            
            ctx.ChangeTracker.DetectChanges();
            Debug.WriteLine(ctx.ChangeTracker.DebugView.LongView);

            var tag = ctx.Tags.Include(x => x.PostTags).ToList();
            Debug.WriteLine(ctx.ChangeTracker.DebugView.LongView);

            ctx.ChangeTracker.DetectChanges();
            Debug.WriteLine(ctx.ChangeTracker.DebugView.LongView);


            ctx.SaveChanges();
        }