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：

/// 
    ///  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    /// 
    public class GroupTableViewModel    {        /// 
        ///  Key        /// 
        public int? Id { get; set; }        /// 
        /// Count        /// 
        public int? Count { get; set; }        /// 
        /// Sum        /// 
        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