如何修复ASP.NET Core Web API子对象创建失败问题？

问题概述 #

在.NET7的ASP.NET Core Web API项目中，BooksController的CreateAsync Post方法无法正常创建Book对象，Swagger返回400验证错误，且不想使用DTO解决该问题。

实体类代码 #

public class Book
{
    public int Id { get; set; }
    public string Title { get; set; }
    public string Author { get; set; }
    public int Count { get; set; }

    // Navigation property
    public BookDetail BookDetail { get; set; }
}

public class BookDetail
{
    public int Id { get; set; }
    public string Description { get; set; }

    // Foreign key
    public int BookId { get; set; }
    // navigation properties
    public Book Book { get; set; }
}

控制器相关代码 #

[HttpPost("Create")]
public async Task<ActionResult<Book>> CreateAsync(Book book)
{
    if (book == null)
    {
        return BadRequest();
    }

    _db.Books.Add(book);

    await _db.SaveChangesAsync();

    return CreatedAtRoute(nameof(GetByIdAsync), new { id = book.Id }, book);
}

[HttpGet("{id:int}")]
public async Task<ActionResult<Book>> GetByIdAsync(int id)
{
    var book = await _db.Books.FindAsync(id);

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

    return Ok(book);
}

Swagger返回的错误信息 #

{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "traceId": "00-2695d8ae6781a68ca32420345ecf40a1-9851e26a3bcb27f2-00",
  "errors": {
    "book": [
      "The book field is required."
    ],
    "$.bookDetail.book": [
      "The JSON value could not be converted to BookShopApp.Models.Book. Path: $.bookDetail.book | LineNumber: 9 | BytePositionInLine: 20."
    ]
  }
}

测试用的JSON输入 #

变体1 #

{
  "id": 0,
  "title": "string",
  "author": "string",
  "count": 0,
  "bookDetail": {
    "id": 0,
    "description": "string",
    "bookId": 0,
    "book": "string"
  }
}

变体2 #

{
  "title": "Book Title",
  "author": "Author Name",
  "count": 3,
  "bookDetail": {
    "description": "Book Description"
  }
}

已尝试的配置（未解决问题） #

// in Program.cs
builder.Services
    .AddControllers()
    .AddJsonOptions(x => x.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles);

解决方案 #

1. 修改实体类的可空导航属性 #

由于.NET7默认启用Nullable参考类型，BookDetail中的Book导航属性被标记为不可空，导致模型绑定验证失败。将其改为可空类型：

public class BookDetail
{
    public int Id { get; set; }
    public string Description { get; set; }

    public int BookId { get; set; }
    // 将导航属性改为可空
    public Book? Book { get; set; }
}

2. 明确控制器参数的绑定来源 #

在CreateAsync方法的参数上添加[FromBody]特性，确保模型绑定从请求体获取JSON数据：

[HttpPost("Create")]
public async Task<ActionResult<Book>> CreateAsync([FromBody] Book book)
{
    // 原有逻辑不变
}

3. 调整JSON输入格式 #

创建对象时无需传递BookDetail中的Book（导航属性）和BookId（外键由EF Core自动关联生成），使用以下正确的JSON格式：

{
  "title": "Book Title",
  "author": "Author Name",
  "count": 3,
  "bookDetail": {
    "description": "Book Description"
  }
}

4. 配置EF Core的实体关系（可选） #

在DbContext的OnModelCreating方法中明确配置一对一关系，确保EF Core正确处理外键关联：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<BookDetail>()
        .HasOne(bd => bd.Book)
        .WithOne(b => b.BookDetail)
        .HasForeignKey<BookDetail>(bd => bd.BookId);
}

完成以上步骤后，重新运行项目，即可正常通过Post请求创建Book对象。

内容的提问来源于stack exchange，提问作者DevMis