Entity Framework связь элементов одной таблицы, чяднт?

[Table("tags")]
    public class Tag
    {
        [ScaffoldColumn(true)]
        [StringLength(60)]
        [Column("id")]
        public string Id { get; set; }

        /* другие поля */

        public virtual ICollection<Tag> ParentTags { get; set; }
        public virtual ICollection<Tag> ChildTags { get; set; }
    }

parent_tag_id character varying(60) NOT NULL,
child_tag_id character varying(60) NOT NULL,

modelBuilder.Entity<Tag>()
                .HasMany<Tag>(a => a.ParentTags)
                .WithMany(a => a.ChildTags)
                .Map(tt =>
                {
                    tt.MapLeftKey("parent_tag_id");
                    tt.MapRightKey("child_tag_id");
                    tt.ToTable("tags_relations");
                });

public ActionResult Edit([Bind(Include ="Id,Title,Visible,Description")] Tag tag,string[] ParentTags)
        {
            if (ModelState.IsValid)
            {
                var parentTags = db.Tags.Where(a => ParentTags.Contains(a.Id)).ToList<Tag>();
                tag.ParentTags = parentTags;
                db.Entry(tag).State = EntityState.Modified;
                
                db.SaveChanges();
            }
            var tags = db.Tags.Where(a => a.Visible == false).Select(c => new {
                Id = c.Id,
                Title = c.Title
            }).ToList();
            ViewBag.Tags = new MultiSelectList(tags, "Id", "Title");

            return View(tag);
        }