asp.net-mvc - 添加多对多关系而不从数据库中获取子实体

Question

我有具有多对多关系的 Book 和 Author 实体，我正在尝试使用数据库优先方法保存关系(连接表中的记录)。

我想从我的工作版本代码开始

 [HttpPost]
        public ActionResult Edit(BookViewModel bookv)
        {
            Mapper.CreateMap<AuthorViewModel, Author>();
            List<Author> authors = Mapper.Map<List<AuthorViewModel>, List<Author>>
(bookv.Authors.ToList());
//remove authors from book object to avoid multiple entities with same key error
           bookv.Authors.Clear();
            Mapper.CreateMap< BookViewModel,Book>();
            Book  book = Mapper.Map<BookViewModel,Book>(bookv);

            db.Books.Attach(book);          
            book.Authors.Clear();
            foreach (Author a in authors)
            {
                //Fetch Author and add relation to book object
                book.Authors.Add(db.Authors.Single(at=>at.AuthorId==a.AuthorId));
            }

            db.ObjectStateManager.ChangeObjectState(book, EntityState.Modified);
            db.SaveChanges();
            return RedirectToAction("Index");

        }

在上面的代码中，我从数据库中获取作者对象(记录)以添加与书对象的关系。而， 我不是从数据库中获取 Book 对象，而是将其附加到上下文中。我不能对 Author 对象做类似的事情吗？

我尝试使用此代码执行此操作，但它首先将新记录添加到 Author 表并添加与书籍和新创建的(不需要的)作者的关系:

 [HttpPost]
    public ActionResult Edit(BookViewModel bookv)
    {
        Mapper.CreateMap<AuthorViewModel, Author>();
        List<Author> authors = Mapper.Map<List<AuthorViewModel>, List<Author>>(bookv.Authors.ToList());
    //   bookv.Authors.Clear();
        Mapper.CreateMap< BookViewModel,Book>();
        Book  book = Mapper.Map<BookViewModel,Book>(bookv);

        foreach (Author a in book.Authors)
        {
            db.Authors.Attach(a);
        }

        db.Books.Attach(book);

        foreach (Author a in authors)
        {
            book.Authors.Add(a);
        }
     db.ObjectStateManager.ChangeObjectState(book, EntityState.Modified);
            db.SaveChanges();
            return RedirectToAction("Index");

        }

Answer 1

[HttpPost]
        public ActionResult Edit(BookViewModel bookv)
        {
            //create maps
            Mapper.CreateMap<AuthorViewModel, Author>();
            Mapper.CreateMap<BookViewModel, Book>();

            //convert view objects to model objects
            Book book = Mapper.Map<BookViewModel, Book>(bookv);
            List<Author> authors = Mapper.Map<List<AuthorViewModel>, List<Author>>   (bookv.Authors.ToList());

            book.Authors.Clear();
            db.Books.Attach(book);


            foreach (Author a in authors) { db.Authors.Attach(a); }

            book.Authors.Clear();

            foreach (Author a in authors) { book.Authors.Add(a); }

            db.ObjectStateManager.ChangeObjectState(book, EntityState.Modified);
            db.SaveChanges();
            return RedirectToAction("Index");

        }