ModelState 无效

Question

我有一个用于图书馆的 ASP.NET Core MVC 项目，我正在尝试创建一个具有来自表单的关系的模型。

我用脚手架功能制作了这个贷款模型和控制器

public class Loan
    {
        [Key]
        public int Id { get; set; }
        [DataType(DataType.Date)]
        public DateTime DueDate { get; set; }
        [ForeignKey("Book")]
        public int BookId { get; set; }
        public virtual Book Book { get; set; }
        [ForeignKey("Reader")]
        public int ReaderId { get; set; }
        public virtual Reader Reader { get; set; }
    }

 public async Task<IActionResult> Create([Bind("Id,DueDate,BookId,ReaderId")] Loan loan)
        {
            if (ModelState.IsValid)
            {
                _context.Add(loan);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            ViewData["BookId"] = new SelectList(_context.Book, "Id", "Id", loan.BookId);
            ViewData["ReaderId"] = new SelectList(_context.Reader, "Id", "Id", loan.ReaderId);
            return View(loan);
        }

<div class="row">
    <div class="col-md-4">
        <form asp-action="Create">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <div class="form-group">
                <label asp-for="DueDate" class="control-label"></label>
                <input asp-for="DueDate" class="form-control" />
                <span asp-validation-for="DueDate" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="BookId" class="control-label"></label>
                <select asp-for="BookId" class ="form-control" asp-items="ViewBag.BookId"></select>
            </div>
            <div class="form-group">
                <label asp-for="ReaderId" class="control-label"></label>
                <select asp-for="ReaderId" class ="form-control" asp-items="ViewBag.ReaderId"></select>
            </div>
            <div class="form-group">
                <input type="submit" value="Create" class="btn btn-primary" />
            </div>
        </form>
    </div>
</div>

当我尝试创建新贷款时，loan.Book 和 loan.Reader 为空。 所以我尝试像这样更改 Create 方法：

public async Task<IActionResult> Create([Bind("Id,DueDate,BookId,ReaderId")] Loan loan)
        {
            loan.Book = _context.Book.FirstOrDefault(b => b.Id == loan.BookId);
            loan.Reader = _context.Reader.FirstOrDefault(r => r.Id == loan.ReaderId);

            if (ModelState.IsValid)
            {
                _context.Add(loan);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            ViewData["BookId"] = new SelectList(_context.Book, "Id", "Id", loan.BookId);
            ViewData["ReaderId"] = new SelectList(_context.Reader, "Id", "Id", loan.ReaderId);
            return View(loan);
        }

它设置了书籍和阅读器，但 ModelState 仍然无效。