我有一个选择框，它使用存储库检索所有类别，但当创建一个想法时，所选类别返回null。
这些是我的模型类：

public class Category
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
}

public class Idea
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public string Rating { get; set; }
    public string Status { get; set; }
    public DateTime Date { get; set; }

    [ForeignKey("Category")]
    public int CategoryId { get; set; }
    public Category? Category { get; set; }
    public  IEnumerable<Category>? Categories { get; set; }
}

这是当我必须创建一个想法时的主控制器，只要我单击创建图标，选择框就会检索所有类别，获取类别ID并将其传递给CategoryId（想法模型中的外键），但它无法将其链接到类别表中的实际ID，因此当显示像@model Idea.Category.Name这样的项目时，它返回null

[HttpPost
public async Task<IActionResult> AddIdea(Idea viewModel)
{
    if (ModelState.IsValid)
    {
        var idea = new Idea
                       {
                           Name = viewModel.Name,
                           Description = viewModel.Description,
                           Rating = viewModel.Rating,
                           Status = viewModel.Status,
                           Date = viewModel.Date,
                           CategoryId = viewModel.CategoryId, 
                           Category = viewModel.Category,  // this is where it says null, without it the modelState becomes false.
                       };

        _ideaRepository.Add(idea);

        // Redirect to home or any other page
        return RedirectToAction("Index"); 
    }

    var categories = await _categoryRepository.GetAllCategories();
    ViewBag.Categories = new SelectList(categories, "Id", "Name");

    return View(viewModel);
}