blah1.js

async function enrollStudents(course, courseId, studentIds) {
  // if (!isPublicGroup(course)) {  // private courses should work fine with default enrolment by omission (since student will never join its own course half-way through)
  //   return;
  // }

  await Promise.all(studentIds.map(async stId => {
    const existingPurchasesForStudent = await dao.find({query: {courseId: courseId, studentIds: stId}});
    const existingEnrolments = await enrolmentsDao.find({query: {userId: stId, courseId: `${course._id}`}});
    if (isStudentReEnrolling(existingEnrolments)) {
      const enrolment = existingEnrolments[0];
      enrolment.dates.push(moment())
      await enrolmentsDao.update(enrolment._id, enrolment, {});
    } else {
      if (existingPurchasesForStudent.length === 0)
        await enrolmentsDao.create({
          creationTime: Date.now(),
          courseId: `${course._id}`,
          userId: stId,
          dates: [new Date()]
        }, {});
    }
  }));
}

function isStudentReEnrolling(existingEnrolments) {
  if (!existingEnrolments.length) return false;
  const dates = existingEnrolments[0].dates;
  const lastDate = dates[dates.length - 1];
  return (dates.length % 2 === 0) && (moment(lastDate) < moment())
}