db.study.find({},{members:1}).forEach(s => {

  for(let i in s.members) {
    delete s.members[i].addedAt;
  }

  db.study.update({_id: s._id},{members: s.members});

});

db.study.find({},{members:1}).forEach(s => {

  for(let i in s.members) {
    delete s.members[i].addedAt;
  }

  db.study.update({_id: s._id},{$set:{members: s.members}});

});

fix mongodb migration script