添加多对多关系而无需从数据库获取子实体

Question

我拥有具有多对多关系的书籍和作者实体，并且我试图使用数据库第一种方法保存关系（联结表中的记录）。

我想先从代码

[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"); 

     } 

我的工作版本在上面的代码中，我从数据库中获取作者的对象（记录）添加与书对象关系。而我不是从数据库中获取Book对象，而是将它附加到上下文中。我能不能用Author对象做类似的事情？

我曾尝试使用此代码要做到这一点，但它第一次增加了新的记录，作者表，并增加了与图书和关系新创建（无用）作者：

[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"); 

     } 

Answer 2

你有没有尝试过呼叫db.Books.Attach(book)db.Authors.Attach(a)循环？

另外，你有没有尝试将书添加到作者？像a.Books.Add(book);而不是？

督察，

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