message: delete courseId
diff: @@ -38,6 +38,9 @@ taskUnitSchema.methods.export = function() {
  delete obj.__v;
  delete obj.updatedAt;
  + // custom properties
  + delete obj._course;
  +
  // "populate" tasks
  const tasks: Array<mongoose.Types.ObjectId> = obj.tasks;
  obj.tasks = [];
  