Update field with another field's value in the document [duplicate]

Answer 1

This can be done through:

db.nameOfCollection.find().forEach(
    function (elem) {
        db.nameOfCollection.update(
            {
                _id: elem._id
            },
            {
                $set: {
                    field2: elem.field1
                }
            }
        );
    }
);