Is it impossible to use Select inside a projection in a union?

xcxc0 0
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    return await result.Select(student => new MarkSheetsStudentByIdDto
    {
        Id = student.RegId,
        FullName = student.FullName,
        AnnualMarkSheets = student.TermOne
        .Select(x => new MarkSheetDto
        {
            ...
            Comments = student.Comments.Where(x => x.StudentId.Equals(student.RegId)).Select(x => x.CommentText).ToList()
        }).Union(student.TermTwo
        .Select(x => new MarkSheetDto
        {
            ...
            Comments = student.Comments.Where(x => x.StudentId.Equals(student.RegId)).Select(x => x.CommentText).ToList()
        })).ToList()
    }).FirstOrDefaultAsync(cancellationToken);

For the above example code snippet, I am getting this error at runtime.

Unable to translate set operation after client projection has been applied. consider moving the set operation before the last 'select' call.

If I remove the comments it will not happen. Can anyone please guide me on how to solve this?

linq entity-framework .net c#
1 Answers

You have to do client-side post processing. EF Core can translate Eager Loading queries if it is simple. Union/Concat adds additional complexity.

var rawResult = await result.Select(student => new 
    {
        Id = student.RegId,
        FullName = student.FullName,
        TermsOne = student.TermOne
          .Select(x => new MarkSheetDto
          {
              ...
              Comments = student.Comments.Where(x => x.StudentId.Equals(student.RegId)).Select(x => x.CommentText).ToList()
          }).ToList(),
        TermsTwo = student.TermTwo
          .Select(x => new MarkSheetDto
          {
              ...
              Comments = student.Comments.Where(x => x.StudentId.Equals(student.RegId)).Select(x => x.CommentText).ToList()
          }).ToList()
    }).FirstOrDefaultAsync(cancellationToken);

if (rawResult == null)
  return null;

return new MarkSheetsStudentByIdDto
{
    Id = rawResult.Id,
    FullName = rawResult.FullName,
    AnnualMarkSheets = rawResult.TermsOne.Concat(rawResult.TermsTwo).ToList()
};
Svyatoslav Danyliv0
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