blah.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: `${course._id}`}});
    if (existingPurchasesForStudent.length === 0 || ensureTheStudentIsEnrolled(existingEnrolments)) {
        await enrolmentsDao.create({
          creationTime: Date.now(),
          courseId: `${course._id}`,
          userId: stId,
          dates: [new Date()]
        }, {});
    }
  }));
}
function ensureTheStudentIsEnrolled(existingEnrolments) {
  return (existingEnrolments.length && existingEnrolments.length % 2 === 0) && (moment(existingEnrolments.pop().creationTime) < moment())
}