“值不能是 null。 參數名稱：source”，同時在視圖中從連接表中添加項目

Question

在我的項目中，我有兩個表：Restaurant(RestaurantId, Name, ...., Adress) 和 RestaurantReviews(ReviewsId, Mark, ...., RestaurantId)。 在我的 PartialView 中，我嘗試顯示餐廳信息並計算平均分數，但如果我嘗試計算平均值，則會出現錯誤：

ArgumentNullException: Value cannot be null. (Parameter 'source')

餐廳模特

public class Restaurant
  {
    [Key]
    public int RestaurantId{ get; set; }

    ....

    [Required(ErrorMessage = "Restaurant name is empty")]
    public string Name{ get; set; }

    ....

    public virtual ICollection<RestaurantReviews> RestaurantReviews{ get; set; }

餐廳評論模型

public class RestaurantReviews
  {
    [Key]
    public int RestaurantReviewsId { get; set; }

    ....

    [Required(ErrorMessage = "Restaurant mark is empty")]
    public int Mark{ get; set; }

    [ForeignKey("Restaurant")]
    public int RestaurantId { get; set; }

    public virtual  Restaurant Restaurant {get; set; }

數據庫上下文

 public DbSet<Restaurant> Restaurant { get; set; }
 public DbSet<RestaurantReviews> RestaurantReviews{ get; set; }

稱為部分視圖

@await Html.PartialAsync("~/Views/Restaurant/RestaurantWidget.cshtml", (IEnumerable<Firma.Data.Data.CMS.Restaurant>)ViewBag.RestaurantWidget)

RestaurantWidget.cshtml

@model IEnumerable<Firma.Data.Data.CMS.Restaurant>


@foreach (var item in Model)
{
    ...
    @item.Name
    ...

    //Without this line everything is ok
    @item.RestaurantReviews.Average(x=>x.Mark)
    //Without this line everything is ok

   ...
}

Controller

public IActionResult Index()
  {
     ViewBag.RestaurantWidget=
        (
           from restaurant in _context.Restaurant
           select restaurant 
           ).Take(5).ToList();
        return View();
  }

附加信息：每家餐廳至少有一個評級。 沒有一家餐廳不會有任何評分

Answer 1

我找到了我的問題的答案，在Controller我改變

public IActionResult Index()
  {
     ViewBag.RestaurantWidget=
        (
           from restaurant in _context.Restaurant
           select restaurant 
           ).Take(5).ToList();
        return View();
  }

在下面編碼

ViewBag.RestaurantWidget= _context.Restaurant.Include(p=>p.RestaurantReviews).Take(4).ToList();