/**
 * Update student records and set their average test score and grade
 * computed from that average.
 */
// update every student record
db.students.updateMany( {}, [
  // Compute the average of all the test scores and set that as the `average` field
  { $set: { average : { $trunc: [ { $avg: "$tests" }, 0 ] } } },
  // Then set the `grade` field based on that average
  { $set: { grade: { $switch: {
    branches: [
      { case: { $gte: [ "$average", 90 ] }, then: "A" },
      { case: { $gte: [ "$average", 80 ] }, then: "B" },
      { case: { $gte: [ "$average", 70 ] }, then: "C" },
      { case: { $gte: [ "$average", 60 ] }, then: "D" }
    ],
    default: "F"
  } } } }
] );