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{ _id: 4, grades: [ { grade: 80, mean: 75, std: 8 }, { grade: 85, mean: 90, std: 5 }, { grade: 85, mean: 85, std: 8 } ]}
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db.students.updateOne( { _id: 4, "grades.grade": 85 }, { $set: { "grades.$.std" : 6 } })