// Update Alice's student record to record class changes
db.students.updateOne( { _id : 'alice' }, [
  // Remove the classes she dropped
  {
    $set : {
      classes : { $setDifference : [ '$classes', [ 'Finance 101' ] ] },
    },
  },
  // Add the classes she's taking intead
  {
    $set : {
      classes  : { $concatArrays : [ '$classes', [
        'Engineering 104',
        'Underwater Basket Weaving',
      ] ] }
    }
  },
] );