为 LINQ 组中的空集合获取 null答案

【问题标题】：Get null for empty collection in LINQ Group为 LINQ 组中的空集合获取 null
【发布时间】：2018-08-04 02:28:28
【问题描述】：

我正在研究 LINQ 查询，部分目标是执行一次 SQL 数据库调用以取得结果。我有很多问题可能有答案。

我需要选择所有问题和答案集合，如果没有特定问题的答案，我仍然需要它。

代码只给我有答案的问题，而不是没有答案的问题

var t3 = (Context.Answers
    .Include(answer => answer.AnswerStatusType)
    .Where(answer => Context.Questions.Where(q => q.profileId == ProfileId)
    .Any(t => t.Id == answer.QuestionId)))
    .GroupBy(
        x => x.QuestionId,
        x => x,
        (key, g) => new
        {
            Question = key,
            Answers = g.ToList(),
        }
    ).ToList();

【问题讨论】：

以Context.Questions 开头，然后...
如果你使用EF，你不应该有一个从Questions到Answers的导航属性吗？如果不是，您不应该使用联接吗？
如何添加加入，考虑到我还需要没有任何答案的问题列表

标签： linq .net-core entity-framework-core

【解决方案1】：

你可能需要这样的东西：

var t3 =
(
    from q in Context.Questions
    where q.profileId == ProfileId
    join a in Context.Answers on q.Id equals a.QuestionId into gas
    select new
    {
        Question = q,
        Answers = gas.ToList(),
    }
).ToList();

【讨论】：

【解决方案2】：

我遇到了完全相同的问题，并用这个巨大的查询解决了它，它可能会被优化，但也许它会帮助有类似问题的人。

        // Perform left join to get all the questions (even those that don't have answers)
        var results = (from quiz in db.Quizzes
                      join question in db.Questions on quiz.QuestionID equals question.QuestionID
                      from answer in db.QuestionAnswers.Where(a => question.QuestionID == a.QuestionID).DefaultIfEmpty()
                      where quiz.QuizID == quizId
                      group (answer != null ? new AnswerVM()
                      {
                          AnswerId = answer.AnswerID,
                          AnswerText = answer.AnswerText,
                          Correct = question.CorrectAnswerID != null ? answer.AnswerID == question.CorrectAnswerID : false,
                      } : null) // For the questions with no answers 
                      by new
                      {
                          question.QuestionID,
                          question.DisplayOrder,
                          question.QuestionText,
                          question.CorrectAnswerID
                      } into g
                      select new QuestionVM
                      {
                          QuestionId = g.Key.QuestionID,
                          QuestionText = g.Key.QuestionText,
                          CorrectAnswerId = g.Key.CorrectAnswerID,
                          Answers = g.ToList()
                      })
                .AsEnumerable()
                .Select(x => {
                    // If there are no answers then group by will return the list with one value (null), 
                    // then we need to change the answers list to be null (not the list with one null value)
                    if (x.Answers.Count(a => a != null) <= 0)
                    {
                        x.Answers = null;
                    }
                    return x;
                }).ToList();

【讨论】：