一开始对这个没怎么重视，只知道efcore的m2m需要中间表，今天做到这块的时候才发现不会写了...查了很多资料，才找到正确写法。赶紧记录一下
首先模型：

    public class Book
    {
        public int BookId { get; set; } 
        public string Title { get; set; }
        public string Description { get; set; } 

       public ICollection<BookAuthor> Authors { get; set; }
    }
    public class Author
    {
        public int AuthorId { get; set; }
        public string Name { get; set; }

        public virtual ICollection<BookAuthor> Books { get; set; }
    }

    public class BookAuthor
    {
        public int BookId { get; set; }       
        public int AuthorId { get; set; } 
    
        public Book Book { get; set; }      
        public Author Author { get; set; }    
    }

    public class EfCoreContext : DbContext
    {
        public DbSet<Book> Books { get; set; }
        public DbSet<Author> Authors { get; set; }
        public DbSet<BookAuthor> BookAuthors { get; set; }


        public EfCoreContext(DbContextOptions<EfCoreContext> options): base(options) { }

        protected override void   OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<BookAuthor>().HasKey(x => new { x.BookId, x.AuthorId });         
        }
    }

C

这个最简单，直接上代码

        [HttpPost]
        public IActionResult Create(BookCreateDTO bookDTO )
        {
            if (bookDTO.Author!=null)
            {
                bookDTO.Book.Authors = new List<BookAuthor>
                {
                    new BookAuthor
                    {
                        Author = bookDTO.Author,
                        Book = bookDTO.Book,
                        Order=0
                    }
                };
            }
          
            _context.Books.Add(bookDTO.Book);
            _context.SaveChanges();
            return Ok();
        }

// BookCreateDTO
    public class BookCreateDTO
    {
        public Book Book { get; set; }
        public Author Author { get; set; }
    }

其实这个Author不要也行，等用户创建完毕后再添加也可以。

R

1. 使用eagar loading的查询（Include）

[HttpGet]
public IActionResult List()
{
    return Ok(_context.Books.Include(x => x.Authors).ThenInclude(y => y.Author).ToList());
}
[HttpGet("{id}")]
public IActionResult One(int id)
{
    return Ok(_context.Books.Include(x => x.Authors).ThenInclude(y => y.Author).FirstOrDefault(x=>x.BookId == id));
}

查询结果：

{
    "bookId": 2,
    "title": "newBook2",
    "description": null,
    "publishedOn": "0001-01-01T00:00:00",
    "publisher": null,
    "price": 0,
    "imageUrl": null,
    "softDeleted": false,
    "authorsLink": [
        {
            "bookId": 2,
            "authorId": 1,
            "order": 0,
            "author": {
                "authorId": 1,
                "name": "Monodev2",
                "bookAuthors": []
            }
        },
        {
            "bookId": 2,
            "authorId": 2,
            "order": 0,
            "author": {
                "authorId": 2,
                "name": "Monodev2",
                "bookAuthors": []
            }
        }
    ]
}

2. 分别查询然后组合

        [HttpGet("{id}")]
        public IActionResult Detail(int id)
        {
            List<Author> authors = _context.BookAuthors
                .Include(x => x.Author)
                .Where(c => c.BookId == id)
                .Select(x=>x.Author)
                .ToList();
            Book book = _context.Books.Single(x => x.BookId == id);
            var result =  new
            {
                book=book,
                authors = authors
            };
            return Ok(result);
        }

上面方法关键在于Select这个地方，因为_context.BookAuthors.Include(x => x.Author).Where(c => c.BookId == id)这样查出来是BookAuthor对象，直接返回客户端数据就会很脏，并且有嵌套，只选择author更符合前端消费习惯

{
    "book": {
        "bookId": 2,
        "title": "newBook2",
        "description": null,
        "publishedOn": "0001-01-01T00:00:00",
        "publisher": null,
        "price": 0,
        "imageUrl": null,
        "softDeleted": false,
        "authors": null
    },
    "authors": [
        {
            "authorId": 1,
            "name": "Monodev2",
            "books": null
        },
        {
            "authorId": 2,
            "name": "Monodev2",
            "books": null
        }
    ]
}

上面两种方法比较明显第二种方法更好。

U

更新是比较麻烦的，首先，需要写一个静态扩展：

    public static class Extensions
    { 

        public static void TryUpdateManyToMany<T, TKey>(this DbContext db, IEnumerable<T> currentItems, IEnumerable<T> newItems, Func<T, TKey> getKey) where T : class
        {
            db.Set<T>().RemoveRange(currentItems.ExceptThat(newItems, getKey));
            db.Set<T>().AddRange(newItems.ExceptThat(currentItems, getKey));
        }

        private static IEnumerable<T> ExceptThat<T, TKey>(this IEnumerable<T> items, IEnumerable<T> other, Func<T, TKey> getKeyFunc)
        {
            return items
                .GroupJoin(other, getKeyFunc, getKeyFunc, (item, tempItems) => new { item, tempItems })
                .SelectMany(t => t.tempItems.DefaultIfEmpty(), (t, temp) => new { t, temp })
                .Where(t => ReferenceEquals(null, t.temp) || t.temp.Equals(default(T)))
                .Select(t => t.t.item);
        }
    }

然后，定义更新模型：

    public class BookUpdateDTO
    {
        public Book Book { get; set; }
        public List<int> Authors { get; set; }
    }

然后，在Controller中这样写：

        [HttpPut]
        public IActionResult Update(BookUpdateDTO bookUpdateDTO)
        {
            var model = _context.Books.Include(x => x.Authors).FirstOrDefault(x => x.BookId == bookUpdateDTO.Book.BookId);
            _context.TryUpdateManyToMany(model.Authors, bookUpdateDTO.Authors
                .Select(x => new BookAuthor
                {
                    AuthorId = x,
                    BookId = bookUpdateDTO.Book.BookId
                }),x=>x.AuthorId);
            _context.SaveChanges();
            return Ok();
        }

本质上，就是遍历传进的model中的MM，然后用静态扩展中的方法进行更新。

D

        [HttpDelete("{id}")]
        public IActionResult Remove(int id)
        {
            var model = _context.Books.FirstOrDefault(x => x.BookId == id);
            if (model != null)
            {
                _context.Books.Remove(model);
                _context.SaveChanges();
                return Ok();
            }
            return NoContent();
        }

EFCore 默认即为级联删除，所以关联表中bookid==2的数据将被一并删除，当然authors不会被删除：

bookid==2的数据已被删除
id=2的author还在

全文完，希望有帮助