Migrations.add({
  version: 4,
  name: "Add display={true} to call history records.",
  up: function() {
    let modifier = { $set: { display: true } };
    CallsHistory.upsert(modifier);
  },
  down: function() {
    let modifier = { $unset: { display } };
    CallsHistory.upsert(modifier);
  }
});