Question

My News.cs类与Comment.cs有一对多的关系，如下所述

public class News
{
    public int NewsId { get; set; }    
    [Display(Name = "Title")]
    public string Title { get; set; }    
    [Display(Name = "Details")]
    public string Details { get; set; }    
    public DateTime DateCreated { get; set; }    
    public int AppUserId { get; set; }
    [ForeignKey("AppUserId")]
    public virtual AppUser AppUser { get; set; }   
    public ICollection<Comment> Comment { get; set; }   
}
public class Comment
{
    public int CommentId { get; set; }
    public string CommentText { get; set; }
    public DateTime DateCreated { get; set; }
    public int AppUserId  { get; set; }
    public int? NewsId { get; set; }
    [ForeignKey("AppUserId")]
    public virtual AppUser AppUser { get; set; }
    [ForeignKey("NewsId")]
    public virtual News News { get; set; }
}

我有一个控制器动作，我试图获取一个新闻项目及其所有评论，所以我设置了两个像这样的viewModels

public class CommentVM
{    
    public string CommentText { get; set; }
    public DateTime DateCreated { get; set; }
    public string Author { get; set; }    
}
public class NewsCommentsVM
{
    [Display(Name = "Title")]
    public string Title { get; set; }
    [Display(Name = "Details")]
    public string Details { get; set; }
    public DateTime DateCreated { get; set; }
    public string Author { get; set; }
    public List<CommentVM> Comments { get; set; }
}

在我的控制器操作中，我有

public ActionResult Details(int? id)
{
    UOW _unit = new UOW();
    if (id == null)
    {
        return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
    }
    News news = _unit.NewsRepository.GetByID(id);

    if (news == null)
    {
        return HttpNotFound();
    }
    var model = new NewsCommentsVM()
    {
        Title = news.Title,
        Details = news.Details,
        DateCreated = news.DateCreated,
        Author = news.AppUser.FirstName
        Comments = news.Comment.Select(c => new CommentVM()
        {
            CommentText = c.CommentText,
            Author = c.AppUser.Email,
            DateCreated = c.DateCreated
        }).ToList()

    };
    return View(result);

}

问题是调试器显示Comment返回Null，而在数据库中有对该特定新闻项的相关注释，因此我收到错误

值不能为空。参数：source

我已经能够在没有问题的情况下在另一个项目中使用此代码。

Answer 1

我认为问题是因为您需要将Comments集合属性更改为virtual。如果您希望延迟加载相关实体，则需要遵循此requirements：

public class News
{
    //...  
    public  virtual ICollection<Comment> Comment { get; set; }   
}

现在，如果您已停用lazy loading，则当您需要查找特定新闻时，其他选项可能会在您的查询中使用Include扩展名方法：

int id=3;
var specificNews=context.News.Include(n=>n.Comment).FirstOrDefault(n=>n.Id==id);

这样，相关实体将包含在查询结果中

实体Sql语句

1 个答案: