LINQ to Entities 无法识别该方法:LastOrDefault

问题描述

概述:在 CompletedQuestions 表中，UserId 对应于完成该问题的用户.Id 属性对应于 Questions 表中的问题之一.我知道，我没有正确指定关系.但我不是很有经验.我只想完成一个项目，然后我会在了解更多信息后回来修复那些糟糕的编码实践.我无法理解以下异常.

Overview:In CompletedQuestions table, UserId corresponds to the user which completed that question. Id property corresponds to one of the questions in the Questions table. I know, i didn't specify relationships properly. But i'm not very experienced. I just want to finish a project, then i will come back and fix those bad coding practices once i learn more. I couldn't understand the following exception.

LINQ to Entities does not recognize the method 'Riddle.Models.CompletedQuestion LastOrDefault[CompletedQuestion](System.Linq.IQueryable`1[Riddle.Models.CompletedQuestion])' method, and this method cannot be translated into a store expression.

    Line 46:                 if (RiddleCompleted == null)
    Line 47:                 {
    Line 48:                     var lastCompletedQuestion = _db.CompletedQuestions.Where(q => q.UserId == currentUserId) // exception occurs in this line
    Line 49:                                                                              .OrderBy(q => q.QuestionNumber)
    Line 50:                                                                              .LastOrDefault();

发生异常的动作:

public ActionResult Riddle(int Id)
        {
            string currentUserId = User.Identity.GetUserId();
            var riddle = _db.Riddles.Where(r => r.Id == Id).Single();

            if (riddle.User.Id == User.Identity.GetUserId())
            {
                var question = riddle.Questions.Where(q => q.QuestionNumber == 1).SingleOrDefault();
                if (question == null)
                {
                    return View("NoMoreQuestions");
                }
                return View("RiddleOwner", question);
            }
            else
            {

                var RiddleCompleted = _db.CompletedRiddles.Where(r => r.Id == Id && r.UserId == currentUserId).SingleOrDefault();
                if (RiddleCompleted == null)
                {
                    var lastCompletedQuestion = _db.CompletedQuestions.Where(q => q.UserId == currentUserId)  // exception occurs in this line
                                                                             .OrderBy(q => q.QuestionNumber)
                                                                             .LastOrDefault();
                    if (lastCompletedQuestion == null)
                    {
                        var question = riddle.Questions.Where(q => q.QuestionNumber == 1).Single();
                        return View(question);
                    }

                    else
                    {
                        var question = riddle.Questions.Where(q => q.QuestionNumber == lastCompletedQuestion.QuestionNumber + 1).SingleOrDefault();
                        return View(question);
                    }
                }
                else
                {
                    return View("NoMoreQuestions");
                }
            }
        }

已完成的问题模型:

public class CompletedQuestion
{
    public int Id { get; set; }
    public string UserId { get; set; }
    public int QuestionNumber { get; set; }
}

问题模型:

  public class Question
    {
        public int Id { get; set; }
        public string Body { get; set; }
        public string Answer { get; set; }
        public Riddle Riddle { get; set; }
        [Column(TypeName ="datetime2")]
        public DateTime CreationDate { get; set; }
        public int QuestionNumber { get; set; }
    }

推荐答案

LastOrDefault() 不受 Linq To Entities 支持.因此，它可以在内存中的集合上工作，但在您尝试查询数据库时则不行.

LastOrDefault() isn't supported by Linq To Entities. So it will work on a collection in memory, but not when you're attempting to query a database .

这是一种有效的处理方式:

This is an efficient way to handle it:

var lastCompletedQuestion =
_db.CompletedQuestions.Where(q => q.UserId == currentUserId)
.OrderByDescending(q => q.QuestionNumber)
.FirstOrDefault()

                                                                         ;

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