router.post('/updateattendance', (req, res) => {

  let attendaceColl = [
                       {roll_number: 9915,date: '2019-05-21',was_present: true},
                       {roll_number: 9904,date: '2019-05-21',was_present: true}
                      ];
  async.eachSeries(attendaceColl, (stdnt, done) => {
    UserSubjectDetails.findOneAndUpdate(
      {
      roll_number: stdnt.roll_number
     },
       {
        $push: {
          'subject_details.attendance.doc' : [{
            date: stdnt.date,
            was_present: stdnt.was_present
          }]
        }

    }, function(err, rsesult) {
      console.log('did')
    }, done);
  }, function(err, res){
    console.log(res)
  });

});