添加多对多的关系，而不从数据库中获取子实体

问题描述

我有Book和Author实体多对多的关系，我想保存数据库的第一种方法的关系（在路口表中的记录）。

我想先从code的工作我的版本

  [HttpPost]
        公众的ActionResult编辑（BookViewModel bookv）
        {
            Mapper.CreateMap＆LT; AuthorViewModel，作者及GT;（）;
            清单＆LT;作者＆GT;作者= Mapper.Map＆LT;名单，LT; AuthorViewModel＆gt;中列出＆LT;作者＆GT;＆GT;
（bookv.Authors.ToList（））;
//从书中删除对象的作者，以避免与同一键错误多个实体
           bookv.Authors.Clear（）;
            Mapper.CreateMap＆LT; BookViewModel，图书＆GT;（）;
            书书= Mapper.Map＆LT; BookViewModel，图书＆GT;（bookv）;            db.Books.Attach（书）;
            book.Authors.Clear（）;
            的foreach（在作者作者一）
            {
                //获取作者，并添加相对于预定目标
                book.Authors.Add（db.Authors.Single（AT = GT; at.AuthorId == a.AuthorId））;
            }            db.ObjectStateManager.ChangeObjectState（书，EntityState.Modified）;
            db.SaveChanges（）;
            返回RedirectToAction（「指数」）;        }
 

在上面code，我从数据库中获取Author对象（记录）添加与书对象关系。然而，我不是从数据库中获取图书的对象，但其附加到上下文。我不能做Author对象相似的地方？

我曾尝试与该code要做到这一点，但它首先增加了新的记录，作者表格，并将与有关的书和新创建的（不必要的）作者：

  [HttpPost]
    公众的ActionResult编辑（BookViewModel bookv）
    {
        Mapper.CreateMap＆LT; AuthorViewModel，作者及GT;（）;
        清单＆LT;作者＆GT;作者= Mapper.Map＆LT;名单，LT; AuthorViewModel＆gt;中列出＆LT;作者＆GT;＆GT;（bookv.Authors.ToList（））;
    // bookv.Authors.Clear（）;
        Mapper.CreateMap＆LT; BookViewModel，图书＆GT;（）;
        书书= Mapper.Map＆LT; BookViewModel，图书＆GT;（bookv）;        的foreach（在book.Authors作者一）
        {
            db.Authors.Attach（一）;
        }        db.Books.Attach（书）;        的foreach（在作者作者一）
        {
            book.Authors.Add（一）;
        }
     db.ObjectStateManager.ChangeObjectState（书，EntityState.Modified）;
            db.SaveChanges（）;
            返回RedirectToAction（「指数」）;        }
 

解决方案

  [HttpPost]
        公众的ActionResult编辑（BookViewModel bookv）
        {
            //创建地图
            Mapper.CreateMap＆LT; AuthorViewModel，作者及GT;（）;
            Mapper.CreateMap＆LT; BookViewModel，图书＆GT;（）;            //转换视图对象模型对象
            书书= Mapper.Map＆LT; BookViewModel，图书＆GT;（bookv）;
            清单＆LT;作者＆GT;作者= Mapper.Map＆LT;名单，LT; AuthorViewModel＆gt;中列出＆LT;作者＆GT;＆GT; （bookv.Authors.ToList（））;            book.Authors.Clear（）;
            db.Books.Attach（书）;
            的foreach（在作者作者一个）{db.Authors.Attach（一）; }            book.Authors.Clear（）;            的foreach（在作者作者一个）{book.Authors.Add（一）; }            db.ObjectStateManager.ChangeObjectState（书，EntityState.Modified）;
            db.SaveChanges（）;
            返回RedirectToAction（「指数」）;        }
 

I have Book and Author entities with many to many relation and I am trying to save the relation (a record in junction table) with Database first approach.

I would like to start with my working version of code

 [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");

        }

In above code, I am fetching Author object (record) from database to add relation with book object. Whereas,I am not fetching Book object from database but attaching it to the context. Can't I do something similar with Author objects?

I have tried to do this with this code but it first adds new records to Author table and adds relation with book and newly created (unwanted) authors:

 [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");

        }

解决方案

[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");

        }

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