Nested LINQ returning a this method cannot be translated into a store expression exception

Question

The following 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();

is giving me this error:

LINQ to Entities does not recognize the method 'System.Collections.Generic.List1[KernWcfTest.DataTransferObjects.JobDetailDto] ToList[JobDetailDto](System.Collections.Generic.IEnumerable1[KernWcfTest.DataTransferObjects.JobDetailDto])' method, and this method cannot be translated into a store expression.

Here are the two dto classes:

[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; }
}

The problem is the sub-select and the JobDetails list. I tried adding the KnownType but it didn't work.

This all works fine in LINQ Pad.

Any ideas?

Cheers

Steve

Answer 1

Don't call ToList on the inner query (the one for JobDetails). The error is "This .ToList method you speak of -- it can't be translated to T-SQL!"

This should work:

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();

Note that you can call ToList on the end of the query, as that part doesn't need to be translated to T-SQL.