/**
 * Switch from separate `firstName` and `lastName` fields to a single
 * `name` field and update the last_updated field at the same time
 */
db.students.updateMany( {}, [
  // Set name field by concatenating the firstName and lastName fields
  { $set : { name : { $concat : [ "$firstName", " ", "$lastName" ] } } },
  // Then remove the firstName/lastName fields
  { $unset : { firstName : 1, lastName : 1 } },
  // Set last_updated to the current date/time
  { $set : { last_updated : "$$NOW" } },
] );