在單個查詢中LINQ查詢GroupBy和Max

Question

我有以下的LINQ查詢，但我想修改它，我想通過staffId分組，並只選擇那些ObservationDate是每個staffId最大值的記錄。

from ob in db.TDTObservations.OfType<TDTSpeedObservation>() 
select new 
{ 
    Id = ob.ID, 
    AcademicYearId = ob.Teachers.FirstOrDefault().Classes.FirstOrDefault().AcademicYearID, 
    observationDate = ob.ObservationDate, 
    schoolId = ob.Teachers.FirstOrDefault().Classes.FirstOrDefault().SchoolID, 
    staffId=ob.Teachers.FirstOrDefault().ID 
}; 

Answer 1

var observations = 
    from ob in db.TDTObservations.OfType<TDTSpeedObservation>() 
    select new { 
     Id = ob.ID, 
     AcademicYearId = ob.Teachers.FirstOrDefault().Classes.FirstOrDefault().AcademicYearID, 
     observationDate = ob.ObservationDate, 
     schoolId = ob.Teachers.FirstOrDefault().Classes.FirstOrDefault().SchoolID, 
     staffId=ob.Teachers.FirstOrDefault().ID 
    }; 

var result = from o in observations 
      group o by o.staffId into g 
      select g.OrderByDescending(x => x.observationDate).First(); 

Answer 2

這個什麼：在此你第一組條目（教師）通過其ID一起，然後從每個組（GRP）你挑選一個與最新ObservationDate

var observations = from d in db.TDTObservations.OfType<TDTSpeedObservation>() 
        group d by d.Teachers.FirstOrDefault().ID into grp 
        select grp.OrderByDescending(g => g.ObservationDate).FirstOrDefault();