为何TryUpdateModelAsync会拒绝外键已设但导航属性为空的模型？

即使已正确设置外键，TryUpdateModelAsync()仍会拒绝导航属性为空的模型对象。比如Book实体的AuthorId已设置但Author为null时，该方法会触发模型验证错误：The Author field is required。

相关实体定义：

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

public class Author
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Book> Books { get; set; }
}

问题根源：当外键AuthorId为非可空值类型（int而非int?）时，Entity Framework会自动将对应的导航属性Author标记为必填项，哪怕你在TryUpdateModelAsync中仅指定更新Title和AuthorId，模型验证依然会检查导航属性是否为空。

方案1：手动移除导航属性的验证错误 #

在TryUpdateModelAsync执行后，直接清除导航属性对应的模型错误，再判断是否允许保存：

public async Task<IActionResult> OnPostAsync(int? id)
{
    var bookToUpdate = await _context.Books.FindAsync(id);
    if (bookToUpdate == null) { return NotFound(); }

    var updateResult = await TryUpdateModelAsync<Book>(
        bookToUpdate, 
        "book", 
        b => b.Title, 
        b => b.AuthorId
    );

    // 移除导航属性的验证错误
    ModelState.Remove("Book.Author");

    if (updateResult && ModelState.IsValid)
    {
        await _context.SaveChangesAsync();
        return RedirectToPage("./Index");
    }
    else 
    {
        foreach (var modelStateKey in ModelState.Keys)
        {
            var modelStateVal = ModelState[modelStateKey];
            foreach (var error in modelStateVal.Errors)
            {
                _logger.LogError($"Key: {modelStateKey}, Error: {error.ErrorMessage}");
            }
        }
    }
    Book = bookToUpdate;
    return RedirectToPage(new { id});
}

方案2：通过Fluent API配置导航属性为可选 #

在DbContext的OnModelCreating方法中，显式配置导航属性为非必填，保留外键的必填约束：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Book>()
        .HasOne(b => b.Author)
        .WithMany(a => a.Books)
        .HasForeignKey(b => b.AuthorId)
        .IsRequired(); // 外键保持必填
        .Navigation(b => b.Author).IsRequired(false); // 导航属性设为非必填
}

方案3：使用DTO避免直接操作实体 #

创建仅包含需更新字段的DTO，绕过实体导航属性的验证逻辑：

public class BookEditDto
{
    public int Id { get; set; }
    public string Title { get; set; }
    public int AuthorId { get; set; }
}

页面模型中改用DTO绑定：

[BindProperty] public BookEditDto BookDto { get; set; }

public async Task<IActionResult> OnPostAsync(int? id)
{
    var bookToUpdate = await _context.Books.FindAsync(id);
    if (bookToUpdate == null) { return NotFound(); }

    bookToUpdate.Title = BookDto.Title;
    bookToUpdate.AuthorId = BookDto.AuthorId;

    if (ModelState.IsValid)
    {
        await _context.SaveChangesAsync();
        return RedirectToPage("./Index");
    }

    Book = bookToUpdate;
    Authors = await _context.Authors.ToListAsync();
    return Page();
}

对应的Razor页面修改为绑定DTO：

<form method="post">
    <input type="hidden" asp-for="BookDto.Id" />
    <input asp-for="BookDto.Title" />
    <select asp-for="BookDto.AuthorId" asp-items="@(new SelectList(Model.Authors,"Id","Name"))"></select>
    <input type="submit" value="Save" />
</form>

内容的提问来源于stack exchange，提问作者Jeremy Salwen