如何将具有多个连接的Linq表达式转换为方法语法或检索选择索引？

Question

我有一个(工作!)linq表达式:

from ca in db.CustomAnswer
join ss in db.SurveySubmission on ca.SubmissionId equals ss.Id
join cq in db.CustomQuestion on ca.QuestionId equals cq.Id
where (ss.SurveyId == request.SurveyId)
orderby ss.Submitted, cq.SortOrder
select new
{
    SubmissionId = ss.Id,
    Answer = ca.Answer
}

我想将select的索引添加到新对象中,例如

from ca in db.CustomAnswer
join ss in db.SurveySubmission on ca.SubmissionId equals ss.Id
join cq in db.CustomQuestion on ca.QuestionId equals cq.Id
where (ss.SurveyId == request.SurveyId)
orderby ss.Submitted, cq.SortOrder
select new
{
    SubmissionId = ss.Id,
    **Code = selectIndex,**
    Answer = ca.Answer
}

为此,我相信我需要先将查询转换为方法语法,以便我可以使用该Select((q, index) => ...)表单.简单来说,我认为它应该是:

  db.SurveySubmission
    .Where(ss => ss.SurveyId == request.SurveyId)
    .OrderBy(ss => ss.Submitted)
    .Join(db.CustomAnswer, ss => ss.Id, ca => ca.SubmissionId, (ss, ca) => new { ss, ca })
    .Join(db.CustomQuestion, o => o.ca.QuestionId, cq => cq.Id, (o, cq) => new { o.ss, o.ca, cq })
    .OrderBy(q => q.cq.SortOrder)
    .Select((q, idx) => new {
                                SubmissionId = q.ss.Id,
                                Answer = q.ca.Answer,
                                Code = idx
                            });

但是,当评估表达式时,我收到一个错误:

LINQ to Entities无法识别方法'System.Linq.IQueryable 1[<>f__AnonymousTypef3 [System.Guid,System.String,System.Int32]]选择[<> f_ AnonymousTypee 3,<>f__AnonymousTypef3](System.Linq.IQueryable 1[<>f__AnonymousTypee3 [My.Data.Namespace] .SurveySubmission,My.Data.Namespace.CustomAnswer,My.Data.Namespace.CustomQuestion]],System.Linq.Expressions.Expression 1[System.Func3 [<> f _AnonymousTypee 3[My.Data.Namespace.SurveySubmission, My.Data.Namespace.CustomAnswer,My.Data.Namespace.CustomQuestion],System.Int32,<>f__AnonymousTypef3 [System.Guid,System.String,System.Int32]]] )'方法,并且此方法无法转换为商店表达式.

我希望这对某人来说是显而易见的吗？我已经盯着它看了好几个小时,我能做出的唯一结论就是我不够聪明......任何人都可以帮忙吗？

Answer 1

EF无法将其转换为SQL,因为在SQL集中是无序的; 索引的想法对它没有任何意义.

相反,千方百计但使用EF查询得到的指数,然后在一个LINQ到对象的查询索引策略:

var query = //your original query goes here

var finalQuery = query.AsEnumerable()
    .Select((answer, index) => new
        {
            answer.SubmissionId,
            answer.Answer,
            Code = index,
        });