models.py

        return super(CourseEnrollmentManager, self).get_queryset().filter(
            course_id=course_id,
            is_active=1,
        ).exclude(user__in=staff).exclude(user__in=admins).exclude(user__in=coaches).count()

    def is_course_full(self, course):
        """
        Returns a boolean value regarding whether a course has already reached it's max enrollment
        capacity
        """
        is_course_full = False
        if course.max_student_enrollments_allowed is not None:
            is_course_full = self.num_enrolled_in_exclude_admins(course.id) >= course.max_student_enrollments_allowed

        return is_course_full

    def users_enrolled_in(self, course_id, include_inactive=False, verified_only=False):
        """
        Return a queryset of User for every user enrolled in the course.

        Arguments:
            course_id (CourseLocator): course_id to return enrollees for.
            include_inactive (boolean): is a boolean when True, returns both active and inactive enrollees
            verified_only (boolean): is a boolean when True, returns only verified enrollees.

        Returns:
            Returns a User queryset.
        """
        filter_kwargs = {
            'courseenrollment__course_id': course_id,
        }
        if not include_inactive:
            filter_kwargs['courseenrollment__is_active'] = True
        if verified_only:
            filter_kwargs['courseenrollment__mode'] = CourseMode.VERIFIED
        return User.objects.filter(**filter_kwargs)

    def enrollment_counts(self, course_id):
        """
        Returns a dictionary that stores the total enrollment count for a course, as well as the
        enrollment count for each individual mode.
        """
        # Unfortunately, Django's "group by"-style queries look super-awkward
        query = use_read_replica_if_available(
            super(CourseEnrollmentManager, self).get_queryset().filter(course_id=course_id, is_active=True).values(
                'mode').order_by().annotate(Count('mode')))
        total = 0
        enroll_dict = defaultdict(int)
        for item in query:
            enroll_dict[item['mode']] = item['mode__count']
            total += item['mode__count']
        enroll_dict['total'] = total
        return enroll_dict

    def enrolled_and_dropped_out_users(self, course_id):
        """Return a queryset of Users in the course."""
        return User.objects.filter(
            courseenrollment__course_id=course_id
        )


# Named tuple for fields pertaining to the state of
# CourseEnrollment for a user in a course.  This type
# is used to cache the state in the request cache.
CourseEnrollmentState = namedtuple('CourseEnrollmentState', 'mode, is_active')


@python_2_unicode_compatible
class CourseEnrollment(models.Model):
    """
    Represents a Student's Enrollment record for a single Course. You should
    generally not manipulate CourseEnrollment objects directly, but use the
    classmethods provided to enroll, unenroll, or check on the enrollment status
    of a given student.

    We're starting to consolidate course enrollment logic in this class, but
    more should be brought in (such as checking against CourseEnrollmentAllowed,
    checking course dates, user permissions, etc.) This logic is currently
    scattered across our views.

    .. no_pii:
    """
    MODEL_TAGS = ['course', 'is_active', 'mode']

    user = models.ForeignKey(User, on_delete=models.CASCADE)

    course = models.ForeignKey(
        CourseOverview,
        db_constraint=False,
        on_delete=models.DO_NOTHING,
    )

    @property
    def course_price(self):
        return get_cosmetic_verified_display_price(self.course)

    @property
    def course_id(self):
        return self._course_id

    @course_id.setter
    def course_id(self, value):
        if isinstance(value, six.string_types):
            self._course_id = CourseKey.from_string(value)
        else:
            self._course_id = value

    created = models.DateTimeField(auto_now_add=True, null=True, db_index=True)

    # If is_active is False, then the student is not considered to be enrolled
    # in the course (is_enrolled() will return False)
    is_active = models.BooleanField(default=True)

    # Represents the modes that are possible. We'll update this later with a
    # list of possible values.
    mode = models.CharField(default=CourseMode.get_default_mode_slug, max_length=100)

    # An audit row will be created for every change to a CourseEnrollment. This
    # will create a new model behind the scenes - HistoricalCourseEnrollment and a
    # table named 'student_courseenrollment_history'.
    history = HistoricalRecords(
        history_id_field=models.UUIDField(default=uuid.uuid4),
        table_name='student_courseenrollment_history'
    )

    objects = CourseEnrollmentManager()

    # cache key format e.g enrollment.<username>.<course_key>.mode = 'honor'
    COURSE_ENROLLMENT_CACHE_KEY = u"enrollment.{}.{}.mode"  # TODO Can this be removed?  It doesn't seem to be used.

    MODE_CACHE_NAMESPACE = u'CourseEnrollment.mode_and_active'

    class Meta(object):
        unique_together = (('user', 'course'), )
        indexes = [Index(fields=['user', '-created'])]
        ordering = ('user', 'course')

    def __init__(self, *args, **kwargs):
        super(CourseEnrollment, self).__init__(*args, **kwargs)

        # Private variable for storing course_overview to minimize calls to the database.
        # When the property .course_overview is accessed for the first time, this variable will be set.
        self._course_overview = None

    def __str__(self):
        return (
            "[CourseEnrollment] {}: {} ({}); active: ({})"
        ).format(self.user, self.course_id, self.created, self.is_active)

    def save(self, force_insert=False, force_update=False, using=None, update_fields=None):
        super(CourseEnrollment, self).save(force_insert=force_insert, force_update=force_update, using=using,
                                           update_fields=update_fields)

        # Delete the cached status hash, forcing the value to be recalculated the next time it is needed.
        cache.delete(self.enrollment_status_hash_cache_key(self.user))

    @classmethod
    def get_or_create_enrollment(cls, user, course_key):
        """
        Create an enrollment for a user in a class. By default *this enrollment
        is not active*. This is useful for when an enrollment needs to go
        through some sort of approval process before being activated. If you
        don't need this functionality, just call `enroll()` instead.

        Returns a CourseEnrollment object.

        `user` is a Django User object. If it hasn't been saved yet (no `.id`
               attribute), this method will automatically save it before
               adding an enrollment for it.

        `course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)

        It is expected that this method is called from a method which has already
        verified the user authentication and access.

        If the enrollment is done due to a CourseEnrollmentAllowed, the CEA will be
        linked to the user being enrolled so that it can't be used by other users.
        """
        # If we're passing in a newly constructed (i.e. not yet persisted) User,
        # save it to the database so that it can have an ID that we can throw
        # into our CourseEnrollment object. Otherwise, we'll get an
        # IntegrityError for having a null user_id.
        assert isinstance(course_key, CourseKey)

        if user.id is None:
            user.save()

        enrollment, __ = cls.objects.get_or_create(
            user=user,
            course_id=course_key,
            defaults={
                'mode': CourseMode.DEFAULT_MODE_SLUG,
                'is_active': False
            }
        )

        # If there was an unlinked CEA, it becomes linked now
        CourseEnrollmentAllowed.objects.filter(
            email=user.email,
            course_id=course_key,
            user__isnull=True
        ).update(user=user)

        return enrollment

    @classmethod
    def get_enrollment(cls, user, course_key, select_related=None):
        """Returns a CourseEnrollment object.

        Args:
            user (User): The user associated with the enrollment.
            course_key (CourseKey): The key of the course associated with the enrollment.

        Returns:
            Course enrollment object or None
        """
        assert user

        if user.is_anonymous:
            return None
        try:
            request_cache = RequestCache('get_enrollment')
            if select_related:
                cache_key = (user.id, course_key, ','.join(select_related))
            else:
                cache_key = (user.id, course_key)
            cache_response = request_cache.get_cached_response(cache_key)
            if cache_response.is_found:
                return cache_response.value

            query = cls.objects
            if select_related is not None:
                query = query.select_related(*select_related)
            enrollment = query.get(
                user=user,
                course_id=course_key
            )
            request_cache.set(cache_key, enrollment)
            return enrollment
        except cls.DoesNotExist:
            return None

    @classmethod
    def is_enrollment_closed(cls, user, course):
        """
        Returns a boolean value regarding whether the user has access to enroll in the course. Returns False if the
        enrollment has been closed.
        """
        # pylint: disable=import-error
        from openedx.core.djangoapps.enrollments.permissions import ENROLL_IN_COURSE
        return not user.has_perm(ENROLL_IN_COURSE, course)

    def update_enrollment(self, mode=None, is_active=None, skip_refund=False):
        """
        Updates an enrollment for a user in a class.  This includes options
        like changing the mode, toggling is_active True/False, etc.

        Also emits relevant events for analytics purposes.

        This saves immediately.

        """
        RequestCache('get_enrollment').clear()

        activation_changed = False
        # if is_active is None, then the call to update_enrollment didn't specify
        # any value, so just leave is_active as it is
        if self.is_active != is_active and is_active is not None:
            self.is_active = is_active
            activation_changed = True

        mode_changed = False
        # if mode is None, the call to update_enrollment didn't specify a new
        # mode, so leave as-is
        if self.mode != mode and mode is not None:
            self.mode = mode
            mode_changed = True

        if activation_changed or mode_changed:
            self.save()
            self._update_enrollment_in_request_cache(
                self.user,
                self.course_id,
                CourseEnrollmentState(self.mode, self.is_active),
            )

        if activation_changed:
            if self.is_active:
                self.emit_event(EVENT_NAME_ENROLLMENT_ACTIVATED)
            else:
                UNENROLL_DONE.send(sender=None, course_enrollment=self, skip_refund=skip_refund)
                self.emit_event(EVENT_NAME_ENROLLMENT_DEACTIVATED)
                self.send_signal(EnrollStatusChange.unenroll)

        if mode_changed:
            # Only emit mode change events when the user's enrollment
            # mode has changed from its previous setting
            self.emit_event(EVENT_NAME_ENROLLMENT_MODE_CHANGED)
            # this signal is meant to trigger a score recalculation celery task,
            # `countdown` is added to celery task as delay so that cohort is duly updated
            # before starting score recalculation
            ENROLLMENT_TRACK_UPDATED.send(
                sender=None,
                user=self.user,
                course_key=self.course_id,
                mode=self.mode,
                countdown=SCORE_RECALCULATION_DELAY_ON_ENROLLMENT_UPDATE,
            )

    def send_signal(self, event, cost=None, currency=None):
        """
        Sends a signal announcing changes in course enrollment status.
        """
        ENROLL_STATUS_CHANGE.send(sender=None, event=event, user=self.user,
                                  mode=self.mode, course_id=self.course_id,
                                  cost=cost, currency=currency)

    @classmethod
    def send_signal_full(cls, event, user=user, mode=mode, course_id=None, cost=None, currency=None):
        """
        Sends a signal announcing changes in course enrollment status.
        This version should be used if you don't already have a CourseEnrollment object
        """
        ENROLL_STATUS_CHANGE.send(sender=None, event=event, user=user,
                                  mode=mode, course_id=course_id,
                                  cost=cost, currency=currency)

    def emit_event(self, event_name):
        """
        Emits an event to explicitly track course enrollment and unenrollment.
        """

        try:
            context = contexts.course_context_from_course_id(self.course_id)
            assert isinstance(self.course_id, CourseKey)
            data = {
                'user_id': self.user.id,
                'course_id': text_type(self.course_id),
                'mode': self.mode,
            }
            segment_properties = {
                'category': 'conversion',
                'label': text_type(self.course_id),
                'org': self.course_id.org,
                'course': self.course_id.course,
                'run': self.course_id.run,
                'mode': self.mode,
            }
            if event_name == EVENT_NAME_ENROLLMENT_ACTIVATED:
                segment_properties['email'] = self.user.email
            with tracker.get_tracker().context(event_name, context):
                tracker.emit(event_name, data)
                segment.track(self.user_id, event_name, segment_properties)

        except:  # pylint: disable=bare-except
            if event_name and self.course_id:
                log.exception(
                    u'Unable to emit event %s for user %s and course %s',
                    event_name,
                    self.user.username,
                    self.course_id,
                )

    @classmethod
    def enroll(cls, user, course_key, mode=None, check_access=False):
        """
        Enroll a user in a course. This saves immediately.

        Returns a CourseEnrollment object.

        `user` is a Django User object. If it hasn't been saved yet (no `.id`
               attribute), this method will automatically save it before
               adding an enrollment for it.

        `course_key` is our usual course_id string (e.g. "edX/Test101/2013_Fall)

        `mode` is a string specifying what kind of enrollment this is. The
               default is the default course mode, 'audit'. Other options
               include 'professional', 'verified', 'honor',
               'no-id-professional' and 'credit'.
               See CourseMode in common/djangoapps/course_modes/models.py.

        `check_access`: if True, we check that an accessible course actually
                exists for the given course_key before we enroll the student.
                The default is set to False to avoid breaking legacy code or
                code with non-standard flows (ex. beta tester invitations), but
                for any standard enrollment flow you probably want this to be True.

        Exceptions that can be raised: NonExistentCourseError,
        EnrollmentClosedError, CourseFullError, AlreadyEnrolledError.  All these
        are subclasses of CourseEnrollmentException if you want to catch all of
        them in the same way.

        It is expected that this method is called from a method which has already
        verified the user authentication.

        Also emits relevant events for analytics purposes.
        """
        if mode is None:
            mode = _default_course_mode(text_type(course_key))
        # All the server-side checks for whether a user is allowed to enroll.
        try:
            course = CourseOverview.get_from_id(course_key)
        except CourseOverview.DoesNotExist:
            # This is here to preserve legacy behavior which allowed enrollment in courses
            # announced before the start of content creation.
            if check_access:
                log.warning(u"User %s failed to enroll in non-existent course %s", user.username, text_type(course_key))
                raise NonExistentCourseError

        if check_access:
            if cls.is_enrollment_closed(user, course):
                log.warning(
                    u"User %s failed to enroll in course %s because enrollment is closed",
                    user.username,
                    text_type(course_key)
                )
                raise EnrollmentClosedError

            if cls.objects.is_course_full(course):
                log.warning(
                    u"Course %s has reached its maximum enrollment of %d learners. User %s failed to enroll.",
                    text_type(course_key),
                    course.max_student_enrollments_allowed,
                    user.username,
                )
                raise CourseFullError
        if cls.is_enrolled(user, course_key):
            log.warning(
                u"User %s attempted to enroll in %s, but they were already enrolled",
                user.username,
                text_type(course_key)
            )
            if check_access:
                raise AlreadyEnrolledError

        # User is allowed to enroll if they've reached this point.
        enrollment = cls.get_or_create_enrollment(user, course_key)
        enrollment.update_enrollment(is_active=True, mode=mode)
        enrollment.send_signal(EnrollStatusChange.enroll)

        return enrollment

    @classmethod
    def enroll_by_email(cls, email, course_id, mode=None, ignore_errors=True):
        """
        Enroll a user in a course given their email. This saves immediately.

        Note that  enrolling by email is generally done in big batches and the
        error rate is high. For that reason, we supress User lookup errors by
        default.

        Returns a CourseEnrollment object. If the User does not exist and
        `ignore_errors` is set to `True`, it will return None.

        `email` Email address of the User to add to enroll in the course.

        `course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)

        `mode` is a string specifying what kind of enrollment this is. The
               default is the default course mode, 'audit'. Other options
               include 'professional', 'verified', 'honor',
               'no-id-professional' and 'credit'.
               See CourseMode in common/djangoapps/course_modes/models.py.

        `ignore_errors` is a boolean indicating whether we should suppress
                        `User.DoesNotExist` errors (returning None) or let it
                        bubble up.

        It is expected that this method is called from a method which has already
        verified the user authentication and access.
        """
        try:
            user = User.objects.get(email=email)
            return cls.enroll(user, course_id, mode)
        except User.DoesNotExist:
            err_msg = u"Tried to enroll email {} into course {}, but user not found"
            log.error(err_msg.format(email, course_id))
            if ignore_errors:
                return None
            raise

    @classmethod
    def unenroll(cls, user, course_id, skip_refund=False):
        """
        Remove the user from a given course. If the relevant `CourseEnrollment`
        object doesn't exist, we log an error but don't throw an exception.

        `user` is a Django User object. If it hasn't been saved yet (no `.id`
               attribute), this method will automatically save it before
               adding an enrollment for it.

        `course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)

        `skip_refund` can be set to True to avoid the refund process.
        """
        RequestCache('get_enrollment').clear()

        try:
            record = cls.objects.get(user=user, course_id=course_id)
            record.update_enrollment(is_active=False, skip_refund=skip_refund)

        except cls.DoesNotExist:
            log.error(
                u"Tried to unenroll student %s from %s but they were not enrolled",
                user,
                course_id
            )

    @classmethod
    def unenroll_by_email(cls, email, course_id):
        """
        Unenroll a user from a course given their email. This saves immediately.
        User lookup errors are logged but will not throw an exception.

        `email` Email address of the User to unenroll from the course.

        `course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)
        """
        RequestCache('get_enrollment').clear()

        try:
            user = User.objects.get(email=email)
            return cls.unenroll(user, course_id)
        except User.DoesNotExist:
            log.error(
                u"Tried to unenroll email %s from course %s, but user not found",
                email,
                course_id
            )

    @classmethod
    def is_enrolled(cls, user, course_key):
        """
        Returns True if the user is enrolled in the course (the entry must exist
        and it must have `is_active=True`). Otherwise, returns False.

        `user` is a Django User object. If it hasn't been saved yet (no `.id`
               attribute), this method will automatically save it before
               adding an enrollment for it.

        `course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)
        """
        enrollment_state = cls._get_enrollment_state(user, course_key)
        return enrollment_state.is_active or False

    @classmethod
    def is_enrolled_by_partial(cls, user, course_id_partial):
        """
        Returns `True` if the user is enrolled in a course that starts with
        `course_id_partial`. Otherwise, returns False.

        Can be used to determine whether a student is enrolled in a course
        whose run name is unknown.

        `user` is a Django User object. If it hasn't been saved yet (no `.id`
               attribute), this method will automatically save it before
               adding an enrollment for it.

        `course_id_partial` (CourseKey) is missing the run component
        """
        assert isinstance(course_id_partial, CourseKey)
        assert not course_id_partial.run  # None or empty string
        course_key = CourseKey.from_string('/'.join([course_id_partial.org, course_id_partial.course, '']))
        querystring = text_type(course_key)
        try:
            return cls.objects.filter(
                user=user,
                course__id__startswith=querystring,
                is_active=1
            ).exists()
        except cls.DoesNotExist:
            return False

    @classmethod
    def enrollment_mode_for_user(cls, user, course_id):
        """
        Returns the enrollment mode for the given user for the given course

        `user` is a Django User object
        `course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)

        Returns (mode, is_active) where mode is the enrollment mode of the student
            and is_active is whether the enrollment is active.
        Returns (None, None) if the courseenrollment record does not exist.
        """
        enrollment_state = cls._get_enrollment_state(user, course_id)
        return enrollment_state.mode, enrollment_state.is_active

    @classmethod
    def enrollments_for_user(cls, user):
        return cls.objects.filter(user=user, is_active=1).select_related('user')

    @classmethod
    def enrollments_for_user_with_overviews_preload(cls, user, courses_limit=None):  # pylint: disable=invalid-name
        """
        List of user's CourseEnrollments, CourseOverviews preloaded if possible.

        We try to preload all CourseOverviews, which are usually lazily loaded
        as the .course_overview property. This is to avoid making an extra
        query for every enrollment when displaying something like the student
        dashboard. If some of the CourseOverviews are not found, we make no
        attempt to initialize them -- we just fall back to existing lazy-load
        behavior. The goal is to optimize the most common case as simply as
        possible, without changing any of the existing contracts.

        The name of this method is long, but was the end result of hashing out a
        number of alternatives, so pylint can stuff it (disable=invalid-name)
        """
        enrollments = cls.enrollments_for_user(user).select_related('schedule', 'course', 'course__image_set')

        if courses_limit:
            return enrollments.order_by('-created')[:courses_limit]
        else:
            return enrollments

    @classmethod
    def enrollment_status_hash_cache_key(cls, user):
        """ Returns the cache key for the cached enrollment status hash.

        Args:
            user (User): User whose cache key should be returned.

        Returns:
            str: Cache key.
        """
        return 'enrollment_status_hash_' + user.username

    @classmethod
    def generate_enrollment_status_hash(cls, user):
        """ Generates a hash encoding the given user's *active* enrollments.

         Args:
             user (User): User whose enrollments should be hashed.

        Returns:
            str: Hash of the user's active enrollments. If the user is anonymous, `None` is returned.
        """
        assert user

        if user.is_anonymous:
            return None

        cache_key = cls.enrollment_status_hash_cache_key(user)
        status_hash = cache.get(cache_key)

        if not status_hash:
            enrollments = cls.enrollments_for_user(user).values_list('course_id', 'mode')
            enrollments = [(six.text_type(e[0]).lower(), e[1].lower()) for e in enrollments]
            enrollments = sorted(enrollments, key=lambda e: e[0])
            hash_elements = [user.username]
            hash_elements += ['{course_id}={mode}'.format(course_id=e[0], mode=e[1]) for e in enrollments]
            status_hash = hashlib.md5('&'.join(hash_elements).encode('utf-8')).hexdigest()

            # The hash is cached indefinitely. It will be invalidated when the user enrolls/unenrolls.
            cache.set(cache_key, status_hash, None)

        return status_hash

    def is_paid_course(self):
        """
        Returns True, if course is paid
        """
        paid_course = CourseMode.is_white_label(self.course_id)
        if paid_course or CourseMode.is_professional_slug(self.mode):
            return True

        return False

    def activate(self):
        """Makes this `CourseEnrollment` record active. Saves immediately."""
        self.update_enrollment(is_active=True)

    def deactivate(self):
        """Makes this `CourseEnrollment` record inactive. Saves immediately. An
        inactive record means that the student is not enrolled in this course.
        """
        self.update_enrollment(is_active=False)

    def change_mode(self, mode):
        """Changes this `CourseEnrollment` record's mode to `mode`.  Saves immediately."""
        self.update_enrollment(mode=mode)

    def refundable(self, user_already_has_certs_for=None):
        """
        For paid/verified certificates, students may always receive a refund if
        this CourseEnrollment's `can_refund` attribute is not `None` (that
        overrides all other rules).

        If the `.can_refund` attribute is `None` or doesn't exist, then ALL of
        the following must be true for this enrollment to be refundable:

            * The user does not have a certificate issued for this course.
            * We are not past the refund cutoff date
            * There exists a 'verified' CourseMode for this course.

        Arguments:
            `user_already_has_certs_for` (set of `CourseKey`):
                 An optional param that is a set of `CourseKeys` that the user
                 has already been issued certificates in.

        Returns:
            bool: Whether is CourseEnrollment can be refunded.
        """
        # In order to support manual refunds past the deadline, set can_refund on this object.
        # On unenrolling, the "UNENROLL_DONE" signal calls CertificateItem.refund_cert_callback(),
        # which calls this method to determine whether to refund the order.
        # This can't be set directly because refunds currently happen as a side-effect of unenrolling.
        # (side-effects are bad)

        if getattr(self, 'can_refund', None) is not None:
            return True

        # If the student has already been given a certificate they should not be refunded
        if user_already_has_certs_for is not None:
            if self.course_id in user_already_has_certs_for:
                return False
        else:
            if GeneratedCertificate.certificate_for_student(self.user, self.course_id) is not None:
                return False

        # If it is after the refundable cutoff date they should not be refunded.
        refund_cutoff_date = self.refund_cutoff_date()
        # `refund_cuttoff_date` will be `None` if there is no order. If there is no order return `False`.
        if refund_cutoff_date is None:
            return False
        if datetime.now(UTC) > refund_cutoff_date:
            return False

        course_mode = CourseMode.mode_for_course(self.course_id, 'verified', include_expired=True)
        if course_mode is None:
            return False
        else:
            return True

    def refund_cutoff_date(self):
        """ Calculate and return the refund window end date. """
        # NOTE: This is here to avoid circular references
        from openedx.core.djangoapps.commerce.utils import ecommerce_api_client, ECOMMERCE_DATE_FORMAT

        date_placed = self.get_order_attribute_value('date_placed')

        if not date_placed:
            order_number = self.get_order_attribute_value('order_number')
            if not order_number:
                return None

            try:
                order = ecommerce_api_client(self.user).orders(order_number).get()
                date_placed = order['date_placed']
                # also save the attribute so that we don't need to call ecommerce again.
                username = self.user.username
                enrollment_attributes = get_enrollment_attributes(username, six.text_type(self.course_id))
                enrollment_attributes.append(
                    {
                        "namespace": "order",
                        "name": "date_placed",
                        "value": date_placed,
                    }
                )
                set_enrollment_attributes(username, six.text_type(self.course_id), enrollment_attributes)
            except HttpClientError:
                log.warning(
                    u"Encountered HttpClientError while getting order details from ecommerce. "
                    u"Order={number} and user {user}".format(number=order_number, user=self.user.id))
                return None

            except HttpServerError:
                log.warning(
                    u"Encountered HttpServerError while getting order details from ecommerce. "
                    u"Order={number} and user {user}".format(number=order_number, user=self.user.id))
                return None

            except SlumberBaseException:
                log.warning(
                    u"Encountered an error while getting order details from ecommerce. "
                    u"Order={number} and user {user}".format(number=order_number, user=self.user.id))
                return None

        refund_window_start_date = max(
            datetime.strptime(date_placed, ECOMMERCE_DATE_FORMAT),
            self.course_overview.start.replace(tzinfo=None)
        )

        return refund_window_start_date.replace(tzinfo=UTC) + EnrollmentRefundConfiguration.current().refund_window

    def get_order_attribute_value(self, attr_name):
        """ Get and return course enrollment order attribute's value."""
        try:
            attribute = self.attributes.get(namespace='order', name=attr_name)
        except ObjectDoesNotExist:
            return None
        except MultipleObjectsReturned:
            # If there are multiple attributes then return the last one.
            enrollment_id = self.get_enrollment(self.user, self.course_id).id
            log.warning(
                u"Multiple CourseEnrollmentAttributes found for user %s with enrollment-ID %s",
                self.user.id,
                enrollment_id
            )
            attribute = self.attributes.filter(namespace='order', name=attr_name).last()

        return attribute.value

    @property
    def username(self):
        return self.user.username

    @property
    def course_overview(self):
        """
        Returns a CourseOverview of the course to which this enrollment refers.
        Returns None if an error occurred while trying to load the course.

        Note:
            If the course is re-published within the lifetime of this
            CourseEnrollment object, then the value of this property will
            become stale.
        """
        if not self._course_overview:
            try:
                self._course_overview = self.course
            except CourseOverview.DoesNotExist:
                log.info('Course Overviews: unable to find course overview for enrollment, loading from modulestore.')
                try:
                    self._course_overview = CourseOverview.get_from_id(self.course_id)
                except (CourseOverview.DoesNotExist, IOError):
                    self._course_overview = None
        return self._course_overview

    @cached_property
    def verified_mode(self):
        return CourseMode.verified_mode_for_course(self.course_id)

    @cached_property
    def upgrade_deadline(self):
        """
        Returns the upgrade deadline for this enrollment, if it is upgradeable.
        If the seat cannot be upgraded, None is returned.
        Note:
            When loading this model, use `select_related` to retrieve the associated schedule object.
        Returns:
            datetime|None
        """
        log.debug('Schedules: Determining upgrade deadline for CourseEnrollment %d...', self.id)
        if not CourseMode.is_mode_upgradeable(self.mode):
            log.debug(
                'Schedules: %s mode of %s is not upgradeable. Returning None for upgrade deadline.',
                self.mode, self.course_id
            )
            return None

        if self.dynamic_upgrade_deadline is not None:
            # When course modes expire they aren't found any more and None would be returned.
            # Replicate that behavior here by returning None if the personalized deadline is in the past.
            if self.dynamic_upgrade_deadline <= datetime.now(UTC):
                log.debug('Schedules: Returning None since dynamic upgrade deadline has already passed.')
                return None

            if self.verified_mode is None or CourseMode.is_professional_mode(self.verified_mode):
                log.debug('Schedules: Returning None for dynamic upgrade deadline since the course does not have a '
                          'verified mode.')
                return None

            return self.dynamic_upgrade_deadline

        return self.course_upgrade_deadline

    @cached_property
    def dynamic_upgrade_deadline(self):
        """
        Returns the learner's personalized upgrade deadline if one exists, otherwise it returns None.

        Note that this will return a value even if the deadline is in the past. This property can be used
        to modify behavior for users with personalized deadlines by checking if it's None or not.

        Returns:
            datetime|None
        """
        if not self.course_overview.self_paced:
            return None

        if not DynamicUpgradeDeadlineConfiguration.is_enabled():
            return None

        course_config = CourseDynamicUpgradeDeadlineConfiguration.current(self.course_id)
        if course_config.opted_out():
            # Course-level config should be checked first since it overrides the org-level config
            return None

        org_config = OrgDynamicUpgradeDeadlineConfiguration.current(self.course_id.org)
        if org_config.opted_out() and not course_config.opted_in():
            return None

        try:
            if not self.schedule or not self.schedule.active:  # pylint: disable=no-member
                return None

            log.debug(
                'Schedules: Pulling upgrade deadline for CourseEnrollment %d from Schedule %d.',
                self.id, self.schedule.id
            )
            return self.schedule.upgrade_deadline
        except ObjectDoesNotExist:
            # NOTE: Schedule has a one-to-one mapping with CourseEnrollment. If no schedule is associated
            # with this enrollment, Django will raise an exception rather than return None.
            log.debug('Schedules: No schedule exists for CourseEnrollment %d.', self.id)
            return None

    @cached_property
    def course_upgrade_deadline(self):
        """
        Returns the expiration datetime for the verified course mode.

        If the mode is already expired, return None. Also return None if the course does not have a verified
        course mode.

        Returns:
            datetime|None
        """
        try:
            if self.verified_mode:
                log.debug('Schedules: Defaulting to verified mode expiration date-time for %s.', self.course_id)
                return self.verified_mode.expiration_datetime
            else:
                log.debug('Schedules: No verified mode located for %s.', self.course_id)
                return None
        except CourseMode.DoesNotExist:
            log.debug('Schedules: %s has no verified mode.', self.course_id)
            return None

    def is_verified_enrollment(self):
        """
        Check the course enrollment mode is verified or not
        """
        return CourseMode.is_verified_slug(self.mode)

    def is_professional_enrollment(self):
        """
        Check the course enrollment mode is professional or not
        """
        return CourseMode.is_professional_slug(self.mode)

    @classmethod
    def is_enrolled_as_verified(cls, user, course_key):
        """
        Check whether the course enrollment is for a verified mode.

        Arguments:
            user (User): The user object.
            course_key (CourseKey): The identifier for the course.

        Returns: bool

        """
        enrollment = cls.get_enrollment(user, course_key)
        return (
            enrollment is not None and
            enrollment.is_active and
            enrollment.is_verified_enrollment()
        )

    @classmethod
    def cache_key_name(cls, user_id, course_key):
        """Return cache key name to be used to cache current configuration.
        Args:
            user_id(int): Id of user.
            course_key(unicode): Unicode of course key

        Returns:
            Unicode cache key
        """
        return cls.COURSE_ENROLLMENT_CACHE_KEY.format(user_id, text_type(course_key))

    @classmethod
    def _get_enrollment_state(cls, user, course_key):
        """
        Returns the CourseEnrollmentState for the given user
        and course_key, caching the result for later retrieval.
        """
        assert user

        if user.is_anonymous:
            return CourseEnrollmentState(None, None)
        enrollment_state = cls._get_enrollment_in_request_cache(user, course_key)
        if not enrollment_state:
            try:
                record = cls.objects.get(user=user, course_id=course_key)
                enrollment_state = CourseEnrollmentState(record.mode, record.is_active)
            except cls.DoesNotExist:
                enrollment_state = CourseEnrollmentState(None, None)
            cls._update_enrollment_in_request_cache(user, course_key, enrollment_state)
        return enrollment_state

    @classmethod
    def bulk_fetch_enrollment_states(cls, users, course_key):
        """
        Bulk pre-fetches the enrollment states for the given users
        for the given course.
        """