不能使用“System.Data.Entity.Core.Objects.ObjectQuery”类型的实例调用

Question

我想通过 userId 查找用户名

此代码 sn-p 工作

Discussion_CreateBy = db.AspNetUsers.Find(discussion.CreatedBy).UserName,

这曾经在以下控制器类中不起作用

Comment_CreateBy = db.AspNetUsers.Find(c.CreatedBy).UserName,

这是我的模型类

public class DiscussionVM
{
    public int Disussion_ID { get; set; }
    public string Discussion_Title { get; set; }
    public string Discussion_Description { get; set; }
    public Nullable<System.DateTime> Discussion_CreateDate { get; set; }
    public string Discussion_CreateBy { get; set; }

    public string Comment_User { get; set; }

    public IEnumerable<CommentVM> Comments { get; set; }

}

public class CommentVM
{

    public int Comment_ID { get; set; }
    public Nullable<System.DateTime> Comment_CreateDate { get; set; }
    public string Comment_CreateBy { get; set; }
    public string Comment_Description { get; set; }

}

这是整个控制器类

    public ActionResult Discussion_Preview()
    {
        int Discussion_ID = 1;

        var discussion = db.AB_Discussion.Where(d => d.Discussion_ID == Discussion_ID).FirstOrDefault();
        var comments = db.AB_DiscussionComments.Where(c => c.Discussion_ID == Discussion_ID);



        DiscussionVM model = new DiscussionVM()
        {

            Disussion_ID = discussion.Discussion_ID,
            Discussion_Title = discussion.Discussion_Name,
            Discussion_Description = discussion.Discussion_Name,
            Discussion_CreateBy = db.AspNetUsers.Find(discussion.CreatedBy).UserName,
            Discussion_CreateDate = discussion.CreatedDate,

            Comments = comments.Select(c => new CommentVM()

            {
                Comment_ID = c.Comment_ID,
                Comment_Description = c.Comment_Discription,
                Comment_CreateBy = db.AspNetUsers.Find(c.CreatedBy).UserName,                    
                Comment_CreateDate = c.CreatedDate

            })

        };



        return View(model);
    }

出现以下错误

在类型“System.Data.Entity.DbSet1[Project.Models.AspNetUser]' cannot be called with instance of type 'System.Data.Entity.Core.Objects.ObjectQuery1[Project.Models.AspNetUser]”上声明的方法“Project.Models.AspNetUser Find(System.Object[])”

Answer 1

Discussion_CreateBy = db.AspNetUsers.Find(discussion.CreatedBy).UserName

之所以有效，是因为讨论是一个内存对象，因为您正在通过调用 FirstOrDefault 来执行查询：

var discussion = db.AB_Discussion.Where(d => d.Discussion_ID == Discussion_ID).FirstOrDefault();

另一方面在下面的语句中：

db.AspNetUsers.Find(c.CreatedBy).UserName

c 还没有被查询到，因为

db.AB_DiscussionComments.Where(c => c.Discussion_ID == Discussion_ID)

返回 IQueriable 而不是实际的 cmets 集合

修复它的最简单方法是将所有 cmets 放入内存（因为无论如何你都需要它们）：

var comments = db.AB_DiscussionComments.Where(c => c.Discussion_ID == Discussion_ID).ToList();