如何为这个连接 3 个实体的 LINQ 使用（左/右）外连接？答案

【问题标题】：How to use (left/right) outer join for this LINQ that join 3 entities?如何为这个连接 3 个实体的 LINQ 使用（左/右）外连接？
【发布时间】：2012-12-08 03:29:06
【问题描述】：

使用 MVC 3、EF4.1：
构建一个测验屏幕，我加入三个实体：步骤、问题、响应

每个步骤可以有很多问题，每个问题可以有一个或没有答案

我的问题是，当我的查询没有答案时，它会返回没有问题的步骤。如何将外部联接（左/右）合并到此 LINQ 中？

var steps = from s in db.Steps
                    join r in db.Responses.Where(x => x.ReviewID == id)
                           on s.StepID equals r.Question.StepID into g
                    orderby s.StepOrder
                    select new Quiz
                    {
                        StepID = s.StepID,
                        Title = s.Title,
                        Results = from x in g
                                  orderby x.Question.DisplayOrder
                                  select new Result
                                  {
                                      QuestionID = x.Question.QuestionID,
                                      DisplayOrder = x.Question.DisplayOrder,
                                      Choices = x.Question.Choices,
                                      ControlType = x.Question.ControlType,
                                      QuestionText = x.Question.QuestionText,
                                      AnswerValue = x.AnswerValue
                                  }
                    };

问题模型：

public class Question
    {
        public Question()
        {
            this.Responses = new List<Response>();
        }

        public int QuestionID { get; set; }
        public string QuestionText { get; set; }
        public Nullable<bool> Required { get; set; }
        public int DisplayOrder { get; set; }
        public int StepID { get; set; }
        public Nullable<int> DataType { get; set; }
        public Nullable<int> ControlType { get; set; }
        public string Choices { get; set; }
        public Nullable<int> MaxLength { get; set; }
        public virtual ICollection<Response> Responses { get; set; }
        public virtual Step Step { get; set; }
        public string NumberedQuestion
        {
            get { return String.Format("{0}. {1}", DisplayOrder, QuestionText); }
        }
    }

回复：

public class Response
    {
        public int ResponseID { get; set; }
        public int UserID { get; set; }
        public int QuestionID { get; set; }
        public string AnswerValue { get; set; }
        public int ReviewID { get; set; }
        public virtual Question Question { get; set; }
    }

步骤：

public Step()
        {
            this.Questions = new List<Question>();
        }

        public int StepID { get; set; }
        public int ReviewID { get; set; }
        public string Title { get; set; }
        public string Description { get; set; }
        public int StepOrder { get; set; }
        public virtual ICollection<Question> Questions { get; set; }

【问题讨论】：

你说过你有三个实体...请与我们分享Question实体...
我在问题中添加了更多细节

标签： asp.net-mvc linq entity-framework outer-join

【解决方案1】：

问题是您从回复中得到问题，所以您只得到有回复的问题。如果你得到每个步骤的问题，然后每个问题的响应，它应该可以工作。

var steps = from s in db.Steps
            orderby s.StepOrder
            select new Quiz
            {
                StepID = s.StepID,
                Title = s.Title,
                Results = from question in s.Questions
                          orderby question.DisplayOrder                              
                          select new Result
                          {
                              QuestionID = question.QuestionID,
                              DisplayOrder = question.DisplayOrder,
                              Choices = question.Choices,
                              ControlType = question.ControlType,
                              QuestionText = question.QuestionText,
                              AnswerValue = question.Responses
                                                    .Where(r => r.ReviewID == id)
                                                    .Select(r => r.AnswerValue)
                                                    .FirstOrDefault()
                          }
            };

【讨论】：

【解决方案2】：

不是很漂亮......但做了它应该做的;）

var result =
    from step in db.Steps

    let questions = 
        db.Questions.Where(item => item.StepID == step.StepID)
    let responses = 
        db.Responses.Where(item => 
                item.ReviewID == id 
                && questions.Any(q => q.QuestionID == item.QuestionID))

    select new Quiz
    {
        StepID = step.StepID,
        Title = step.Title,
        Results =
            from question in questions.OrderBy(item => item.DisplayOrder)
            select new Result
            {
                QuestionID = question.QuestionID,
                DisplayOrder = question.DisplayOrder,
                Choices = question.Choices,
                ControlType = question.ControlType,
                QuestionText = question.QuestionText,
                AnswerValue = 
                    responses.Any(item => item.QuestionID == question.QuestionID)
                    ? responses.First(item => 
                        item.QuestionID == question.QuestionID).AnswerValue
                    : null
            }
    };

【讨论】：