c# - 具有外键关系的 ASP.NET Core Web API 和 EF Core 模型

Question

我正在开发一个用于教育目的的基本 Web API 项目，但我在处理 EF 模型关系时遇到了问题。我有 2 个模型。 消息 和消息板。

public class Message
    {
        public long Id { get; set; }
        public string Text { get; set; }
        public string User { get; set; }
        public DateTime PostedDate { get; set; }

        public long MessageBoardId { get; set; }
        [ForeignKey("MessageBoardId")]
        public MessageBoard MessageBoard { get; set; }
    }

public class MessageBoard
    {
        public long Id { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }

        public ICollection<Message> Messages { get; set; }
    }

我已经设置了我的 DBContext 并创建了一个迁移来配置数据库。我使用 EF 脚手架生成了两个 Web API Controller 。迁移似乎正确地检测了两个模型之间的关系:

modelBuilder.Entity("Asp.net_Core_Web_Api.Models.Message", b =>
{
   b.HasOne("Asp.net_Core_Web_Api.Models.MessageBoard", "MessageBoard")
   .WithMany("Messages")
   .HasForeignKey("MessageBoardId")
   .OnDelete(DeleteBehavior.Cascade);
});

但是，当我创建一个 MessageBoard，然后使用 MessageBoard 的 ID 创建一条消息时，它们似乎没有正确链接。在 PostMan 中，我正在执行以下操作:

1) 发布一个新的留言板

POST - https://localhost:44384/api/MessageBoards/

主体 - 原始 - Json

{
 "Name":"Test Board",
 "Description":"A Message board for testing purposes."
}

返回

{
    "id": 4,
    "name": "Test Board",
    "description": "A Message board for testing purposes.",
    "messages": null
}

2) 发布新消息

POST - https://localhost:44384/api/Messages

主体 - 原始 - JSON

{
    "Text":"Posting my first message!",
    "User":"Jesse",
    "PostedDate":"1/1/2019",

    "MessageBoardId":4
}

返回

{
    "id": 2,
    "text": "Posting my first message!",
    "user": "Jesse",
    "postedDate": "2019-01-01T00:00:00",
    "messageBoardId": 4,
    "messageBoard": null
}

I would expect that the messageBoard would not be null, and it would instead return the JSON for the messageBoard that was previously created. If I change to a GET method, it is also null. Why is it null?

编辑:这是我的 Controller 。我删除了除 GET 和 POST 之外的操作。

[Route("api/[controller]")]
[ApiController]
public class MessageBoardsController : ControllerBase
{
        private readonly MessageBoardContext _context;

        public MessageBoardsController(MessageBoardContext context)
        {
            _context = context;
        }

        // GET: api/MessageBoards/5
        [HttpGet("{id}")]
        public async Task<ActionResult<MessageBoard>> GetMessageBoard(long id)
        {
            var messageBoard = await _context.MessageBoards.FindAsync(id);

            if (messageBoard == null)
            {
                return NotFound();
            }

            return messageBoard;
        }


        // POST: api/MessageBoards
        [HttpPost]
        public async Task<ActionResult<MessageBoard>> PostMessageBoard(MessageBoard messageBoard)
        {
            _context.MessageBoards.Add(messageBoard);
            await _context.SaveChangesAsync();

            return CreatedAtAction("GetMessageBoard", new { id = messageBoard.Id }, messageBoard);
        }
}


[Route("api/[controller]")]
[ApiController]
public class MessagesController : ControllerBase
{
        private readonly MessageBoardContext _context;

        public MessagesController(MessageBoardContext context)
        {
            _context = context;
        }

        // GET: api/Messages/5
        [HttpGet("{id}")]
        public async Task<ActionResult<Message>> GetMessage(long id)
        {
            var message = await _context.Messages.FindAsync(id);

            if (message == null)
            {
                return NotFound();
            }

            return message;
        }


        // POST: api/Messages
        [HttpPost]
        public async Task<ActionResult<Message>> PostMessage(Message message)
        {
            _context.Messages.Add(message);
            await _context.SaveChangesAsync();

            return CreatedAtAction("GetMessage", new { id = message.Id }, message);
        }
}

Answer 1

您需要load related data

例如，对于您的 MessageBoard GET - //GET: api/MessageBoards/5

更改自:

var messageBoard = await _context.MessageBoards.FindAsync(id);

到

var messageBoard = await _context.MessageBoards
                        .Include(i=>i.Messages)
                        .FirstOrDefaultAsync(i => i.Id == id);