c# - 返回此方法的嵌套 LINQ 无法转换为存储表达式异常

Question

以下 LINQ:

retval = ( from jm in entities.JobMasters
                 where jm.UserId == userId && jm.IsRemote == false
                 select new JobDto
                 {
                     JobMasterId = jm.JobMasterId,
                     ExternalTaskId = jm.ExternalTaskId,
                     JobDetails = ( from jd in entities.JobDetails
                                    where jd.JobMasterId == jm.JobMasterId
                                    select new JobDetailDto { ScreenFieldId = jd.ScreenFieldId, FieldValue = jd.FieldValue }
                     ).ToList()
                 }
            ).ToList();

给我这个错误:

LINQ to Entities does not recognize the method 'System.Collections.Generic.List`1[KernWcfTest.DataTransferObjects.JobDetailDto] ToList[JobDetailDto](System.Collections.Generic.IEnumerable`1[KernWcfTest.DataTransferObjects.JobDetailDto])' method, and this method cannot be translated into a store expression.

这是两个 dto 类:

[DataContract]
public class JobDetailDto
{
    [DataMember]
    public int ScreenFieldId { get; set; }

    [DataMember]
    public string FieldValue { get; set; }
}

[DataContract]
[KnownType(typeof(JobDetailDto))]
public class JobDto
{
    [DataMember]
    public int JobMasterId { get; set; }

    [DataMember]
    public string ExternalTaskId { get; set; }

    [DataMember]
    public List<JobDetailDto> JobDetails { get; set; }
}

问题是子选择和 JobDetails 列表。我尝试添加 KnownType 但它没有用。

这在 LINQ Pad 中一切正常。

有什么想法吗？

干杯

史蒂夫

Answer 1

不要在内部查询(用于 JobDetails 的查询)上调用 ToList。错误是“您所说的这个 .ToList 方法——它无法转换为 T-SQL!”

这应该有效:

retval = ( from jm in entities.JobMasters
             where jm.UserId == userId && jm.IsRemote == false
             select new JobDto
             {
                 JobMasterId = jm.JobMasterId,
                 ExternalTaskId = jm.ExternalTaskId,
                 JobDetails =   from jd in entities.JobDetails
                                where jd.JobMasterId == jm.JobMasterId
                                select new JobDetailDto { ScreenFieldId = jd.ScreenFieldId, FieldValue = jd.FieldValue }
                 )
             }
        ).ToList();

请注意，您可以在查询结束时调用 ToList，因为该部分不需要转换为 T-SQL。