实体框架嵌套查询Select()问题 [英] Entity Framework Nested Query Select() Problem

问题描述

我是实体框架的新成员，并尝试使用 Answers 表中的外键获取数据，但出现错误

I am new to the entity framework and trying to get data using foreign key from Answers table but I am getting error

不支持嵌套查询.操作1 =案例"Operation2 =收集"

The nested query is not supported. Operation1='Case' Operation2='Collect'

p.Answers.Count()或 p.Answers.SingleOrDefault().correct_answer 作为字符串可以正常工作，但 p.Answers.Select(c => c.correct_answer).ToList()引发嵌套查询错误

p.Answers.Count() or p.Answers.SingleOrDefault().correct_answeras string works fine but p.Answers.Select(c => c.correct_answer).ToList() throwing nested query error

我想将其保留在一个查询中，因为可能有成千上万的问题，所以我不想检查单独查询的答案.以下是我的代码.

I want to keep it in one query because there could be thousands of questions so I don't want to check answers to the separate queries. Following is my code.

return db.Questions.Where(p => p.q_id == q_id).Select(p => new QuestionViewModel
{
   q_id = p.q_id,
   q_text = p.q_text,
   q_answer = p.Answers.Count() > 0 ? p.Answers.Select(c => c.correct_answer).ToList() : null
}).OrderBy(x => x.q_id).ToList();

ViewModel

public class QuestionViewModel
{
    public long q_id { get; set; }
    public string q_text { get; set; }
    public List<string> q_answer { get; set; }
}

推荐答案

例外是告诉您EF不支持条件子查询，该表达式由以下表达式暗示:

The exception is telling you that EF does not support conditional subquery, implied by expressions like this:

p.Answers.Count() > 0 ? p.Answers.Select(c => c.correct_answer).ToList() : null

因此只需删除条件运算符:

So simply remove the conditional operator:

q_answer = p.Answers.Select(c => c.correct_answer).ToList()

如果没有相关问题的答案， q_answer 将填充 empty 列表，而不是 null ，这是正常的(预期)集合类型对象的行为.

In case there are no related answers for question, q_answer will be populated with empty list rather than null, which is the normal (expected) behavior for collection type objects.

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