Linq Join Group by 时报Nullable object must have a value.

例如:

                     from s in subject on ch.SubId equals s.SubId
join gc in (from aq in question
group aq by aq.ChapterId
into gaq
select new
{
Id = gaq.Key,
Count = gaq.Count(),
})
on s.QueId equals gc.Id
into gc2
from gc in gc2.DefaultIfEmpty()

结果将会报错

生成的sql语句符合预期,为简单的join

解决方法:

	/// <summary>
/// 解决问题: efcore group new dynamic对象时生成int 型而不是 int? 而导致 Nullable object must have a value.的问题
/// </summary>
public class GroupTableViewModel
{
/// <summary>
/// Key
/// </summary>
public int? Id { get; set; }
/// <summary>
/// Count
/// </summary>
public int? Count { get; set; }
/// <summary>
/// Sum
/// </summary>
public int? Sum { get; set; } public int? Max { get; set; }
}

加上 GroupTableViewModel 问题解决

                     from s in subject on ch.SubId equals s.SubId
join gc in (from aq in question
group aq by aq.ChapterId
into gaq
select new GroupTableViewModel
{
Id = gaq.Key,
Count = gaq.Count(),
})
on s.QueId equals gc.Id
into gc2
from gc in gc2.DefaultIfEmpty()

解决Join 多值报错

                                from employee in employees
join student in students
on new { employee.FirstName, employee.LastName } equals new { student.FirstName, student.LastName }
select employee.FirstName + " " + employee.LastName;

例如

                                      from u in User
join sac in (from sa in Answer
join e in Exam on sa.ExamId equals e.ExamId
group sa by new { UserId = sa.UserId, ExamId = sa.ExamId, }
into gsa
select new
{
UserId = gsa.Key.UserId,
ExamId = gsa.Key.ExamId,
ScoreCount = gsa.Sum(x => x.Score),
})
on new { UserId = u.UserId, ExamId = ue.ExamId } equals new { UserId = sac.UserId, ExamId = sac.ExamId }
into sac2
from sac in sac2.DefaultIfEmpty()

生成sql语句正常 但报错 Nullable object must have a value.

解决方法:

    public class GroupExam
{
public int? UserId { get; set; }
public int? ExamId { get; set; }
public decimal? ShortAnswerScoreCount { get; set; }
}

将原linq修改为

                                      from u in User
join sac in (from sa in Answer
join e in Exam on sa.ExamId equals e.ExamId
group sa by new { UserId = sa.UserId, ExamId = sa.ExamId, }
into gsa
select new GroupExam
{
UserId = gsa.Key.UserId,
ExamId = gsa.Key.ExamId,
ScoreCount = gsa.Sum(x => x.Score),
})
on new { UserId = u.UserId, ExamId = ue.ExamId } equals new { UserId = (int)sac.UserId, ExamId = sac.ExamId }
into sac2
from sac in sac2.DefaultIfEmpty()

问题解决

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