如何在 ASP.NET CORE WEB API 中實現多對多關系

Question

所以我有4節課：

    public class Book
    {
      public int BookId { get; set; }
      public string Title { get; set; }
      public Author Author { get; set; }
      public int AuthorId { get; set; }
      public ICollection<BookCategory> BookCategories { get; set; }
    }  

    public class Author
    {
      public int AuthorId { get; set; }
      public string Name { get; set; }
      public string Surname { get; set; }
      public ICollection<Book> Books { get; set; }
    }  

    public class Category
    {
      public int CategoryId { get; set; }
      public string CategoryName { get; set; }
      public ICollection<BookCategory> BookCategories { get; set; }
    }

    public class BookCategory
    {
      public int BookId { get; set; }
      public Book Book { get; set; }
      public int CategoryId { get; set; }
      public Category Category { get; set; }
    }

我想為 [HttpGet] 請求編寫 Function ，以便它像這樣返回 JSON ：

[
  {
     "bookId": "1",
     "title": "Book Title 1",
     "authorName": "Author Name 1",
     "aurhorSurname": "Author Surname 1",
     "categories": ["category1", "category2"]
  },
  {
     "bookId": "2",
     "title": "Book Title 2",
     "authorName": "Author Name 1",
     "aurhorSurname": "Author Surname 1",
     "categories": ["category2", "category3"]
  },
  {
     "bookId": "3",
     "title": "Book Title 3",
     "authorName": "Author Name 1",
     "aurhorSurname": "Author Surname 1",
     "categories": ["category1", "category4"]
  },
]

我知道如何處理一對多的關系，我為作者和書籍創建了 API，每個作者都有他寫的一系列書籍。 但是這個連接表（BookCategory）讓我感到困惑。

Answer 1

對於返回的 json 格式，可以先通過Include和ThenInclude獲取所有關聯數據。

然后使用Select方法和新對象組成返回的數據結構：

        public IActionResult ReturnJson()
        { 
            var data = _context.Book.Include(x => x.Author)
                .Include(x => x.BookCategories).ThenInclude(x => x.Category)
                .Select(x => new
                {
                    bookId = x.BookId,
                    title = x.Title,
                    authorName = x.Author.Name,
                    authorSurname = x.Author.Surname,
                    categories = x.BookCategories.Select(k => k.Category.CategoryName).ToList()
                });
            return Json(data);
        }

這是返回的數據：