Course M101 - Help to solve Homework 2.2

gistfile1.js

db.grades.aggregate({
    '$group': {'_id': '$student_id'}}).forEach(function(i) {
        var x = db.grades.find({ student_id: i._id, type: 'homework' }).sort({ score: 1 })[0];
        db.grades.remove({ _id: x._id });
    }
);