EF 将新的多记录添加到表中

上下文类这是我的上下文类。它的工作原理是在博客帖子标签表中添加帖子的 ID 和标签的 ID......但是，它会添加新的标记 ID，而不是已存储在数据库中的现有标记。

public class WebsiteDBContext : DbContext
{
    public DbSet<BlogPost> BlogPosts { get; set; }
    public DbSet<Tag> Tags { get; set; }
    public DbSet<Author> Authors { get; set; }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<BlogPost>().HasRequired(bp => bp.Author).WithMany(a => a.BlogPosts);
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<BlogPost>().HasMany(bp => bp.Tags).WithMany(t => t.Posts).Map(m =>
        {
            m.MapLeftKey("BlogPostID");
            m.MapRightKey("TagID");
            m.ToTable("BlogPostTags");
        });
    }
}

我想发生什么：

Tag Table                         BlogPostTags Table
ID  Name                          BlogID    TagID
----------------                  ----------------
1   Dogs                            1         1
2   Cats                            1         9
3   Birds                           1         11
4   Horses
5   Rabbits
6   Reptiles
7   Insects
8   Nature
9   Puppies
10  Kittens
11  Cute
12  Products

但真正发生的是它会在标签表中创建带有 ID 的新标签。这样

Tag Table                         BlogPostTags Table
ID  Name                          BlogID    TagID
----------------                  ----------------
1   Dogs                            1         13
2   Cats                            1         14
3   Birds                           1         15
4   Horses
5   Rabbits
6   Reptiles
7   Insects
8   Nature
9   Puppies
10  Kittens
11  Cute
12  Products
13  Dogs
14  Puppies
15  Cute

重复 名字不好！ 如何解决此问题以获得预期的结果？这是我的动作结果方法

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create([Bind(Include = "Title,PublishDate,PublishTime,Body,AuthorID,Tags")] CreateBlogPostVM blogpost)
{
    List<Tag> tags = new List<Tag>();
    foreach (var tag in blogpost.Tags.Where(t => t.IsChecked == true))
    {
        tags.Add(new Tag { TagID = tag.TagID, Name = tag.Name });
    }
    if (ModelState.IsValid)
    {
        BlogPost newPost = new BlogPost
        {
            Title = blogpost.Title,
            PublishDate = blogpost.PublishDate,
            PublishTime = blogpost.PublishTime,
            Body = blogpost.Body,
            Author = db.Authors.Where(bp => bp.AuthorID == blogpost.AuthorID).Single(),
            Tags = tags //Add list of tags 
        };
        db.BlogPosts.Add(newPost);
        db.SaveChanges();
        return RedirectToAction("Index");
    }
    return View(blogpost);
}

        foreach (var tag in blogpost.Tags.Where(t => t.IsChecked == true))
        {
            tags.Add(new Tag { TagID = tag.TagID, Name = tag.Name });
        }

        foreach (var tag in blogpost.Tags.Where(t => t.IsChecked == true))
        {
            tags.Add(db.Tags.Find(tag.TagID));
        }