models.py

"""
Models for User Information (students, staff, etc)

Migration Notes

If you make changes to this model, be sure to create an appropriate migration
file and check it in at the same time as your model changes. To do that,

1. Go to the edx-platform dir
2. ./manage.py lms schemamigration student --auto description_of_your_change
3. Add the migration file created in edx-platform/common/djangoapps/student/migrations/
"""


import hashlib
import json
import logging
import uuid
from collections import OrderedDict, defaultdict, namedtuple
from datetime import datetime, timedelta
from functools import total_ordering
from importlib import import_module

import six
from config_models.models import ConfigurationModel
from course_modes.models import CourseMode, get_cosmetic_verified_display_price
from django.apps import apps
from django.conf import settings
from django.contrib.auth.models import User
from django.contrib.auth.signals import user_logged_in, user_logged_out
from django.contrib.sites.models import Site
from django.core.cache import cache
from django.core.exceptions import MultipleObjectsReturned, ObjectDoesNotExist
from django.core.validators import FileExtensionValidator, RegexValidator
from django.db import IntegrityError, models
from django.db.models import Count, Q, Index
from django.db.models.signals import post_save, pre_save
from django.db.utils import ProgrammingError
from django.dispatch import receiver
from django.utils.encoding import python_2_unicode_compatible
from django.utils.functional import cached_property
from django.utils.translation import ugettext_lazy as _
from django.utils.translation import ugettext_noop
from django_countries.fields import CountryField
from edx_django_utils.cache import RequestCache
from edx_rest_api_client.exceptions import SlumberBaseException
from eventtracking import tracker
from model_utils.models import TimeStampedModel
from opaque_keys.edx.django.models import CourseKeyField
from opaque_keys.edx.keys import CourseKey
from pytz import UTC
from simple_history.models import HistoricalRecords
from six import text_type
from six.moves import range
from six.moves.urllib.parse import urlencode
from slumber.exceptions import HttpClientError, HttpServerError
from student.signals import ENROLL_STATUS_CHANGE, ENROLLMENT_TRACK_UPDATED, UNENROLL_DONE
from track import contexts, segment
from user_util import user_util
from util.milestones_helpers import is_entrance_exams_enabled
from util.model_utils import emit_field_changed_events, get_changed_fields_dict
from util.query import use_read_replica_if_available

import openedx.core.djangoapps.django_comment_common.comment_client as cc
from lms.djangoapps.certificates.models import GeneratedCertificate
from lms.djangoapps.courseware.models import (
    CourseDynamicUpgradeDeadlineConfiguration,
    DynamicUpgradeDeadlineConfiguration,
    OrgDynamicUpgradeDeadlineConfiguration
)
from lms.djangoapps.verify_student.models import SoftwareSecurePhotoVerification
from openedx.core.djangoapps.content.course_overviews.models import CourseOverview
from openedx.core.djangoapps.enrollments.api import (
    _default_course_mode,
    get_enrollment_attributes,
    set_enrollment_attributes
)
from openedx.core.djangoapps.site_configuration import helpers as configuration_helpers
from openedx.core.djangoapps.xmodule_django.models import NoneToEmptyManager
from openedx.core.djangolib.model_mixins import DeletableByUserValue

log = logging.getLogger(__name__)
AUDIT_LOG = logging.getLogger("audit")
SessionStore = import_module(settings.SESSION_ENGINE).SessionStore  # pylint: disable=invalid-name

# enroll status changed events - signaled to email_marketing.  See email_marketing.tasks for more info


# ENROLL signal used for free enrollment only
class EnrollStatusChange(object):
    """
    Possible event types for ENROLL_STATUS_CHANGE signal
    """
    # enroll for a course
    enroll = 'enroll'
    # unenroll for a course
    unenroll = 'unenroll'
    # add an upgrade to cart
    upgrade_start = 'upgrade_start'
    # complete an upgrade purchase
    upgrade_complete = 'upgrade_complete'
    # add a paid course to the cart
    paid_start = 'paid_start'
    # complete a paid course purchase
    paid_complete = 'paid_complete'

UNENROLLED_TO_ALLOWEDTOENROLL = u'from unenrolled to allowed to enroll'
ALLOWEDTOENROLL_TO_ENROLLED = u'from allowed to enroll to enrolled'
ENROLLED_TO_ENROLLED = u'from enrolled to enrolled'
ENROLLED_TO_UNENROLLED = u'from enrolled to unenrolled'
UNENROLLED_TO_ENROLLED = u'from unenrolled to enrolled'
ALLOWEDTOENROLL_TO_UNENROLLED = u'from allowed to enroll to enrolled'
UNENROLLED_TO_UNENROLLED = u'from unenrolled to unenrolled'
DEFAULT_TRANSITION_STATE = u'N/A'
SCORE_RECALCULATION_DELAY_ON_ENROLLMENT_UPDATE = 30

TRANSITION_STATES = (
    (UNENROLLED_TO_ALLOWEDTOENROLL, UNENROLLED_TO_ALLOWEDTOENROLL),
    (ALLOWEDTOENROLL_TO_ENROLLED, ALLOWEDTOENROLL_TO_ENROLLED),
    (ENROLLED_TO_ENROLLED, ENROLLED_TO_ENROLLED),
    (ENROLLED_TO_UNENROLLED, ENROLLED_TO_UNENROLLED),
    (UNENROLLED_TO_ENROLLED, UNENROLLED_TO_ENROLLED),
    (ALLOWEDTOENROLL_TO_UNENROLLED, ALLOWEDTOENROLL_TO_UNENROLLED),
    (UNENROLLED_TO_UNENROLLED, UNENROLLED_TO_UNENROLLED),
    (DEFAULT_TRANSITION_STATE, DEFAULT_TRANSITION_STATE)
)


class AnonymousUserId(models.Model):
    """
    This table contains user, course_Id and anonymous_user_id

    Purpose of this table is to provide user by anonymous_user_id.

    We generate anonymous_user_id using md5 algorithm,
    and use result in hex form, so its length is equal to 32 bytes.

    .. no_pii: We store anonymous_user_ids here, but do not consider them PII under OEP-30.
    """

    objects = NoneToEmptyManager()

    user = models.ForeignKey(User, db_index=True, on_delete=models.CASCADE)
    anonymous_user_id = models.CharField(unique=True, max_length=32)
    course_id = CourseKeyField(db_index=True, max_length=255, blank=True)


def anonymous_id_for_user(user, course_id, save=True):
    """
    Return a unique id for a (user, course) pair, suitable for inserting
    into e.g. personalized survey links.

    If user is an `AnonymousUser`, returns `None`

    Keyword arguments:
    save -- Whether the id should be saved in an AnonymousUserId object.
    """
    # This part is for ability to get xblock instance in xblock_noauth handlers, where user is unauthenticated.
    assert user

    if user.is_anonymous:
        return None

    cached_id = getattr(user, '_anonymous_id', {}).get(course_id)
    if cached_id is not None:
        return cached_id

    # include the secret key as a salt, and to make the ids unique across different LMS installs.
    hasher = hashlib.md5()
    hasher.update(settings.SECRET_KEY.encode('utf8'))
    hasher.update(text_type(user.id).encode('utf8'))
    if course_id:
        hasher.update(text_type(course_id).encode('utf-8'))
    digest = hasher.hexdigest()

    if not hasattr(user, '_anonymous_id'):
        user._anonymous_id = {}  # pylint: disable=protected-access

    user._anonymous_id[course_id] = digest  # pylint: disable=protected-access

    if save is False:
        return digest

    try:
        AnonymousUserId.objects.get_or_create(
            user=user,
            course_id=course_id,
            anonymous_user_id=digest,
        )
    except IntegrityError:
        # Another thread has already created this entry, so
        # continue
        pass

    return digest


def user_by_anonymous_id(uid):
    """
    Return user by anonymous_user_id using AnonymousUserId lookup table.

    Do not raise `django.ObjectDoesNotExist` exception,
    if there is no user for anonymous_student_id,
    because this function will be used inside xmodule w/o django access.
    """

    if uid is None:
        return None

    request_cache = RequestCache('user_by_anonymous_id')
    cache_response = request_cache.get_cached_response(uid)
    if cache_response.is_found:
        return cache_response.value

    try:
        user = User.objects.get(anonymoususerid__anonymous_user_id=uid)
        request_cache.set(uid, user)
        return user
    except ObjectDoesNotExist:
        request_cache.set(uid, None)
        return None


def is_username_retired(username):
    """
    Checks to see if the given username has been previously retired
    """
    locally_hashed_usernames = user_util.get_all_retired_usernames(
        username,
        settings.RETIRED_USER_SALTS,
        settings.RETIRED_USERNAME_FMT
    )

    # TODO: Revert to this after username capitalization issues detailed in
    # PLAT-2276, PLAT-2277, PLAT-2278 are sorted out:
    # return User.objects.filter(username__in=list(locally_hashed_usernames)).exists()

    # Avoid circular import issues
    from openedx.core.djangoapps.user_api.models import UserRetirementStatus

    # Sandbox clean builds attempt to create users during migrations, before the database
    # is stable so UserRetirementStatus may not exist yet. This workaround can also go
    # when we are done with the username updates.
    try:
        return User.objects.filter(username__in=list(locally_hashed_usernames)).exists() or \
            UserRetirementStatus.objects.filter(original_username=username).exists()
    except ProgrammingError as exc:
        # Check the error message to make sure it's what we expect
        if "user_api_userretirementstatus" in text_type(exc):
            return User.objects.filter(username__in=list(locally_hashed_usernames)).exists()
        raise


def username_exists_or_retired(username):
    """
    Check a username for existence -or- retirement against the User model.
    """
    return User.objects.filter(username=username).exists() or is_username_retired(username)


def is_email_retired(email):
    """
    Checks to see if the given email has been previously retired
    """
    locally_hashed_emails = user_util.get_all_retired_emails(
        email,
        settings.RETIRED_USER_SALTS,
        settings.RETIRED_EMAIL_FMT
    )

    return User.objects.filter(email__in=list(locally_hashed_emails)).exists()


def email_exists_or_retired(email):
    """
    Check an email against the User model for existence.
    """
    return User.objects.filter(email=email).exists() or is_email_retired(email)


def get_retired_username_by_username(username):
    """
    If a UserRetirementStatus object with an original_username matching the given username exists,
    returns that UserRetirementStatus.retired_username value.  Otherwise, returns a "retired username"
    hashed using the newest configured salt.
    """
    UserRetirementStatus = apps.get_model('user_api', 'UserRetirementStatus')
    try:
        status = UserRetirementStatus.objects.filter(original_username=username).order_by('-modified').first()
        if status:
            return status.retired_username
    except UserRetirementStatus.DoesNotExist:
        pass
    return user_util.get_retired_username(username, settings.RETIRED_USER_SALTS, settings.RETIRED_USERNAME_FMT)


def get_retired_email_by_email(email):
    """
    If a UserRetirementStatus object with an original_email matching the given email exists,
    returns that UserRetirementStatus.retired_email value.  Otherwise, returns a "retired email"
    hashed using the newest configured salt.
    """
    UserRetirementStatus = apps.get_model('user_api', 'UserRetirementStatus')
    try:
        status = UserRetirementStatus.objects.filter(original_email=email).order_by('-modified').first()
        if status:
            return status.retired_email
    except UserRetirementStatus.DoesNotExist:
        pass
    return user_util.get_retired_email(email, settings.RETIRED_USER_SALTS, settings.RETIRED_EMAIL_FMT)


def _get_all_retired_usernames_by_username(username):
    """
    Returns a generator of "retired usernames", one hashed with each
    configured salt. Used for finding out if the given username has
    ever been used and retired.
    """
    return user_util.get_all_retired_usernames(username, settings.RETIRED_USER_SALTS, settings.RETIRED_USERNAME_FMT)


def _get_all_retired_emails_by_email(email):
    """
    Returns a generator of "retired emails", one hashed with each
    configured salt. Used for finding out if the given email has
    ever been used and retired.
    """
    return user_util.get_all_retired_emails(email, settings.RETIRED_USER_SALTS, settings.RETIRED_EMAIL_FMT)


def get_potentially_retired_user_by_username(username):
    """
    Attempt to return a User object based on the username, or if it
    does not exist, then any hashed username salted with the historical
    salts.
    """
    locally_hashed_usernames = list(_get_all_retired_usernames_by_username(username))
    locally_hashed_usernames.append(username)
    potential_users = User.objects.filter(username__in=locally_hashed_usernames)

    # Have to disambiguate between several Users here as we could have retirees with
    # the same username, but for case.
    # If there's only 1 we're done, this should be the common case
    if len(potential_users) == 1:
        return potential_users[0]

    # No user found, throw the usual error
    if not potential_users:
        raise User.DoesNotExist()

    # For a brief period, users were able to retire accounts and make another account with
    # the same differently-cased username, like "testuser" and "TestUser".
    # If there are two users found, return the one that's the *actual* case-matching username,
    # whether retired or not.
    if len(potential_users) == 2:
        # Figure out which user has been retired.
        if potential_users[0].username.startswith(settings.RETIRED_USERNAME_PREFIX):
            retired = potential_users[0]
            active = potential_users[1]
        else:
            retired = potential_users[1]
            active = potential_users[0]

        # If the active (non-retired) user's username doesn't *exactly* match (including case),
        # then the retired account must be the one that exactly matches.
        return active if active.username == username else retired

    # We should have, at most, a retired username and an active one with a username
    # differing only by case. If there are more we need to disambiguate them by hand.
    raise Exception('Expected 1 or 2 Users, received {}'.format(text_type(potential_users)))


def get_potentially_retired_user_by_username_and_hash(username, hashed_username):
    """
    To assist in the retirement process this method will:
    - Confirm that any locally hashed username matches the passed in one
      (in case of salt mismatches with the upstream script).
    - Attempt to return a User object based on the username, or if it
      does not exist, the any hashed username salted with the historical
      salts.
    """
    locally_hashed_usernames = list(_get_all_retired_usernames_by_username(username))

    if hashed_username not in locally_hashed_usernames:
        raise Exception('Mismatched hashed_username, bad salt?')

    locally_hashed_usernames.append(username)
    return User.objects.get(username__in=locally_hashed_usernames)


class UserStanding(models.Model):
    """
    This table contains a student's account's status.
    Currently, we're only disabling accounts; in the future we can imagine
    taking away more specific privileges, like forums access, or adding
    more specific karma levels or probationary stages.

    .. no_pii:
    """
    ACCOUNT_DISABLED = u"disabled"
    ACCOUNT_ENABLED = u"enabled"
    USER_STANDING_CHOICES = (
        (ACCOUNT_DISABLED, u"Account Disabled"),
        (ACCOUNT_ENABLED, u"Account Enabled"),
    )

    user = models.OneToOneField(User, db_index=True, related_name='standing', on_delete=models.CASCADE)
    account_status = models.CharField(
        blank=True, max_length=31, choices=USER_STANDING_CHOICES
    )
    changed_by = models.ForeignKey(User, blank=True, on_delete=models.CASCADE)
    standing_last_changed_at = models.DateTimeField(auto_now=True)


class UserProfile(models.Model):
    """This is where we store all the user demographic fields. We have a
    separate table for this rather than extending the built-in Django auth_user.

    Notes:
        * Some fields are legacy ones from the first run of 6.002, from which
          we imported many users.
        * Fields like name and address are intentionally open ended, to account
          for international variations. An unfortunate side-effect is that we
          cannot efficiently sort on last names for instance.

    Replication:
        * Only the Portal servers should ever modify this information.
        * All fields are replicated into relevant Course databases

    Some of the fields are legacy ones that were captured during the initial
    MITx fall prototype.

    .. pii: Contains many PII fields. Retired in AccountRetirementView.
    .. pii_types: name, location, birth_date, gender, biography, phone_number
    .. pii_retirement: local_api
    """
    # cache key format e.g user.<user_id>.profile.country = 'SG'
    PROFILE_COUNTRY_CACHE_KEY = u"user.{user_id}.profile.country"

    class Meta(object):
        db_table = "auth_userprofile"
        permissions = (("can_deactivate_users", "Can deactivate, but NOT delete users"),)

    # CRITICAL TODO/SECURITY
    # Sanitize all fields.
    # This is not visible to other users, but could introduce holes later
    user = models.OneToOneField(User, unique=True, db_index=True, related_name='profile', on_delete=models.CASCADE)
    name = models.CharField(blank=True, max_length=255, db_index=True)

    meta = models.TextField(blank=True)  # JSON dictionary for future expansion
    courseware = models.CharField(blank=True, max_length=255, default=u'course.xml')

    # Language is deprecated and no longer used. Old rows exist that have
    # user-entered free form text values (ex. "English"), some of which have
    # non-ASCII values. You probably want UserPreference version of this, which
    # stores the user's preferred language code. See openedx/core/djangoapps/lang_pref
    # for more information.
    language = models.CharField(blank=True, max_length=255, db_index=True)

    # Location is no longer used, but is held here for backwards compatibility
    # for users imported from our first class.
    location = models.CharField(blank=True, max_length=255, db_index=True)

    # Optional demographic data we started capturing from Fall 2012
    this_year = datetime.now(UTC).year
    VALID_YEARS = list(range(this_year, this_year - 120, -1))
    year_of_birth = models.IntegerField(blank=True, null=True, db_index=True)
    GENDER_CHOICES = (
        (u'm', ugettext_noop(u'Male')),
        (u'f', ugettext_noop(u'Female')),
        # Translators: 'Other' refers to the student's gender
        (u'o', ugettext_noop(u'Other/Prefer Not to Say'))
    )
    gender = models.CharField(
        blank=True, null=True, max_length=6, db_index=True, choices=GENDER_CHOICES
    )

    # [03/21/2013] removed these, but leaving comment since there'll still be
    # p_se and p_oth in the existing data in db.
    # ('p_se', 'Doctorate in science or engineering'),
    # ('p_oth', 'Doctorate in another field'),
    LEVEL_OF_EDUCATION_CHOICES = (
        (u'p', ugettext_noop(u'Doctorate')),
        (u'm', ugettext_noop(u"Master's or professional degree")),
        (u'b', ugettext_noop(u"Bachelor's degree")),
        (u'a', ugettext_noop(u"Associate degree")),
        (u'hs', ugettext_noop(u"Secondary/high school")),
        (u'jhs', ugettext_noop(u"Junior secondary/junior high/middle school")),
        (u'el', ugettext_noop(u"Elementary/primary school")),
        # Translators: 'None' refers to the student's level of education
        (u'none', ugettext_noop(u"No formal education")),
        # Translators: 'Other' refers to the student's level of education
        (u'other', ugettext_noop(u"Other education"))
    )
    level_of_education = models.CharField(
        blank=True, null=True, max_length=6, db_index=True,
        choices=LEVEL_OF_EDUCATION_CHOICES
    )
    mailing_address = models.TextField(blank=True, null=True)
    city = models.TextField(blank=True, null=True)
    country = CountryField(blank=True, null=True)
    goals = models.TextField(blank=True, null=True)
    allow_certificate = models.BooleanField(default=1)
    bio = models.CharField(blank=True, null=True, max_length=3000, db_index=False)
    profile_image_uploaded_at = models.DateTimeField(null=True, blank=True)
    phone_regex = RegexValidator(regex=r'^\+?1?\d*$', message="Phone number can only contain numbers.")
    phone_number = models.CharField(validators=[phone_regex], blank=True, null=True, max_length=50)

    @property
    def has_profile_image(self):
        """
        Convenience method that returns a boolean indicating whether or not
        this user has uploaded a profile image.
        """
        return self.profile_image_uploaded_at is not None

    @property
    def age(self):
        """ Convenience method that returns the age given a year_of_birth. """
        year_of_birth = self.year_of_birth
        year = datetime.now(UTC).year
        if year_of_birth is not None:
            return self._calculate_age(year, year_of_birth)

    @property
    def level_of_education_display(self):
        """ Convenience method that returns the human readable level of education. """
        if self.level_of_education:
            return self.__enumerable_to_display(self.LEVEL_OF_EDUCATION_CHOICES, self.level_of_education)

    @property
    def gender_display(self):
        """ Convenience method that returns the human readable gender. """
        if self.gender:
            return self.__enumerable_to_display(self.GENDER_CHOICES, self.gender)

    def get_meta(self):  # pylint: disable=missing-docstring
        js_str = self.meta
        if not js_str:
            js_str = dict()
        else:
            js_str = json.loads(self.meta)

        return js_str

    def set_meta(self, meta_json):  # pylint: disable=missing-docstring
        self.meta = json.dumps(meta_json)

    def set_login_session(self, session_id=None):
        """
        Sets the current session id for the logged-in user.
        If session_id doesn't match the existing session,
        deletes the old session object.
        """
        meta = self.get_meta()
        old_login = meta.get('session_id', None)
        if old_login:
            SessionStore(session_key=old_login).delete()
        meta['session_id'] = session_id
        self.set_meta(meta)
        self.save()

    def requires_parental_consent(self, date=None, age_limit=None, default_requires_consent=True):
        """Returns true if this user requires parental consent.

        Args:
            date (Date): The date for which consent needs to be tested (defaults to now).
            age_limit (int): The age limit at which parental consent is no longer required.
                This defaults to the value of the setting 'PARENTAL_CONTROL_AGE_LIMIT'.
            default_requires_consent (bool): True if users require parental consent if they
                have no specified year of birth (default is True).

        Returns:
             True if the user requires parental consent.
        """
        if age_limit is None:
            age_limit = getattr(settings, 'PARENTAL_CONSENT_AGE_LIMIT', None)
            if age_limit is None:
                return False

        # Return True if either:
        # a) The user has a year of birth specified and that year is fewer years in the past than the limit.
        # b) The user has no year of birth specified and the default is to require consent.
        #
        # Note: we have to be conservative using the user's year of birth as their birth date could be
        # December 31st. This means that if the number of years since their birth year is exactly equal
        # to the age limit then we have to assume that they might still not be old enough.
        year_of_birth = self.year_of_birth
        if year_of_birth is None:
            return default_requires_consent

        if date is None:
            age = self.age
        else:
            age = self._calculate_age(date.year, year_of_birth)

        return age < age_limit

    def __enumerable_to_display(self, enumerables, enum_value):
        """ Get the human readable value from an enumerable list of key-value pairs. """
        return dict(enumerables)[enum_value]

    def _calculate_age(self, year, year_of_birth):
        """Calculate the youngest age for a user with a given year of birth.

        :param year: year
        :param year_of_birth: year of birth
        :return: youngest age a user could be for the given year
        """
        # There are legal implications regarding how we can contact users and what information we can make public
        # based on their age, so we must take the most conservative estimate.
        return year - year_of_birth - 1

    @classmethod
    def country_cache_key_name(cls, user_id):
        """Return cache key name to be used to cache current country.
        Args:
            user_id(int): Id of user.

        Returns:
            Unicode cache key
        """
        return cls.PROFILE_COUNTRY_CACHE_KEY.format(user_id=user_id)


@receiver(models.signals.post_save, sender=UserProfile)
def invalidate_user_profile_country_cache(sender, instance, **kwargs):  # pylint:   disable=unused-argument, invalid-name
    """Invalidate the cache of country in UserProfile model. """

    changed_fields = getattr(instance, '_changed_fields', {})

    if 'country' in changed_fields:
        cache_key = UserProfile.country_cache_key_name(instance.user_id)
        cache.delete(cache_key)
        log.info("Country changed in UserProfile for %s, cache deleted", instance.user_id)


@receiver(pre_save, sender=UserProfile)
def user_profile_pre_save_callback(sender, **kwargs):
    """
    Ensure consistency of a user profile before saving it.
    """
    user_profile = kwargs['instance']

    # Remove profile images for users who require parental consent
    if user_profile.requires_parental_consent() and user_profile.has_profile_image:
        user_profile.profile_image_uploaded_at = None

    # Cache "old" field values on the model instance so that they can be
    # retrieved in the post_save callback when we emit an event with new and
    # old field values.
    user_profile._changed_fields = get_changed_fields_dict(user_profile, sender)


@receiver(post_save, sender=UserProfile)
def user_profile_post_save_callback(sender, **kwargs):
    """
    Emit analytics events after saving the UserProfile.
    """
    user_profile = kwargs['instance']
    # pylint: disable=protected-access
    emit_field_changed_events(
        user_profile,
        user_profile.user,
        sender._meta.db_table,
        excluded_fields=['meta']
    )


@receiver(pre_save, sender=User)
def user_pre_save_callback(sender, **kwargs):
    """
    Capture old fields on the user instance before save and cache them as a
    private field on the current model for use in the post_save callback.
    """
    user = kwargs['instance']
    user._changed_fields = get_changed_fields_dict(user, sender)


@receiver(post_save, sender=User)
def user_post_save_callback(sender, **kwargs):
    """
    When a user is modified and either its `is_active` state or email address
    is changed, and the user is, in fact, active, then check to see if there
    are any courses that it needs to be automatically enrolled in.

    Additionally, emit analytics events after saving the User.
    """
    user = kwargs['instance']

    changed_fields = user._changed_fields

    if 'is_active' in changed_fields or 'email' in changed_fields:
        if user.is_active:
            ceas = CourseEnrollmentAllowed.for_user(user).filter(auto_enroll=True)

            for cea in ceas:
                enrollment = CourseEnrollment.enroll(user, cea.course_id)

                manual_enrollment_audit = ManualEnrollmentAudit.get_manual_enrollment_by_email(user.email)
                if manual_enrollment_audit is not None:
                    # get the enrolled by user and reason from the ManualEnrollmentAudit table.
                    # then create a new ManualEnrollmentAudit table entry for the same email
                    # different transition state.
                    ManualEnrollmentAudit.create_manual_enrollment_audit(
                        manual_enrollment_audit.enrolled_by,
                        user.email,
                        ALLOWEDTOENROLL_TO_ENROLLED,
                        manual_enrollment_audit.reason,
                        enrollment
                    )

    # Because `emit_field_changed_events` removes the record of the fields that
    # were changed, wait to do that until after we've checked them as part of
    # the condition on whether we want to check for automatic enrollments.
    # pylint: disable=protected-access
    emit_field_changed_events(
        user,
        user,
        sender._meta.db_table,
        excluded_fields=['last_login', 'first_name', 'last_name'],
        hidden_fields=['password']
    )


class UserSignupSource(models.Model):
    """
    This table contains information about users registering
    via Micro-Sites

    .. no_pii:
    """
    user = models.ForeignKey(User, db_index=True, on_delete=models.CASCADE)
    site = models.CharField(max_length=255, db_index=True)


def unique_id_for_user(user, save=True):
    """
    Return a unique id for a user, suitable for inserting into
    e.g. personalized survey links.

    Keyword arguments:
    save -- Whether the id should be saved in an AnonymousUserId object.
    """
    # Setting course_id to '' makes it not affect the generated hash,
    # and thus produce the old per-student anonymous id
    return anonymous_id_for_user(user, None, save=save)


# TODO: Should be renamed to generic UserGroup, and possibly
# Given an optional field for type of group
class UserTestGroup(models.Model):
    """
    .. no_pii:
    """
    users = models.ManyToManyField(User, db_index=True)
    name = models.CharField(blank=False, max_length=32, db_index=True)
    description = models.TextField(blank=True)


class Registration(models.Model):
    """
    Allows us to wait for e-mail before user is registered. A
    registration profile is created when the user creates an
    account, but that account is inactive. Once the user clicks
    on the activation key, it becomes active.

    .. no_pii:
    """

    class Meta(object):
        db_table = "auth_registration"

    user = models.OneToOneField(User, on_delete=models.CASCADE)
    activation_key = models.CharField((u'activation key'), max_length=32, unique=True, db_index=True)

    def register(self, user):
        # MINOR TODO: Switch to crypto-secure key
        self.activation_key = uuid.uuid4().hex
        self.user = user
        self.save()

    def activate(self):
        self.user.is_active = True
        self.user.save(update_fields=['is_active'])
        log.info(u'User %s (%s) account is successfully activated.', self.user.username, self.user.email)


class PendingNameChange(DeletableByUserValue, models.Model):
    """
    This model keeps track of pending requested changes to a user's email address.

    .. pii: Contains new_name, retired in LMSAccountRetirementView
    .. pii_types: name
    .. pii_retirement: local_api
    """
    user = models.OneToOneField(User, unique=True, db_index=True, on_delete=models.CASCADE)
    new_name = models.CharField(blank=True, max_length=255)
    rationale = models.CharField(blank=True, max_length=1024)


class PendingEmailChange(DeletableByUserValue, models.Model):
    """
    This model keeps track of pending requested changes to a user's email address.

    .. pii: Contains new_email, retired in AccountRetirementView
    .. pii_types: email_address
    .. pii_retirement: local_api
    """
    user = models.OneToOneField(User, unique=True, db_index=True, on_delete=models.CASCADE)
    new_email = models.CharField(blank=True, max_length=255, db_index=True)
    activation_key = models.CharField((u'activation key'), max_length=32, unique=True, db_index=True)

    def request_change(self, email):
        """Request a change to a user's email.

        Implicitly saves the pending email change record.

        Arguments:
            email (unicode): The proposed new email for the user.

        Returns:
            unicode: The activation code to confirm the change.

        """
        self.new_email = email
        self.activation_key = uuid.uuid4().hex
        self.save()
        return self.activation_key


class PendingSecondaryEmailChange(DeletableByUserValue, models.Model):
    """
    This model keeps track of pending requested changes to a user's secondary email address.

    .. pii: Contains new_secondary_email, not currently retired
    .. pii_types: email_address
    .. pii_retirement: retained
    """
    user = models.OneToOneField(User, unique=True, db_index=True, on_delete=models.CASCADE)
    new_secondary_email = models.CharField(blank=True, max_length=255, db_index=True)
    activation_key = models.CharField((u'activation key'), max_length=32, unique=True, db_index=True)


EVENT_NAME_ENROLLMENT_ACTIVATED = 'edx.course.enrollment.activated'
EVENT_NAME_ENROLLMENT_DEACTIVATED = 'edx.course.enrollment.deactivated'
EVENT_NAME_ENROLLMENT_MODE_CHANGED = 'edx.course.enrollment.mode_changed'


@python_2_unicode_compatible
class LoginFailures(models.Model):
    """
    This model will keep track of failed login attempts.

    .. no_pii:
    """
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    failure_count = models.IntegerField(default=0)
    lockout_until = models.DateTimeField(null=True)

    @classmethod
    def _get_record_for_user(cls, user):
        """
        Gets a user's record, and fixes any duplicates that may have arisen due to get_or_create
        race conditions. See https://code.djangoproject.com/ticket/13906 for details.

        Use this method in place of `LoginFailures.objects.get(user=user)`
        """
        records = LoginFailures.objects.filter(user=user).order_by('-lockout_until')
        for extra_record in records[1:]:
            extra_record.delete()
        return records.get()

    @classmethod
    def is_feature_enabled(cls):
        """
        Returns whether the feature flag around this functionality has been set
        """
        return settings.FEATURES['ENABLE_MAX_FAILED_LOGIN_ATTEMPTS']

    @classmethod
    def is_user_locked_out(cls, user):
        """
        Static method to return in a given user has his/her account locked out
        """
        try:
            record = cls._get_record_for_user(user)
            if not record.lockout_until:
                return False

            now = datetime.now(UTC)
            until = record.lockout_until
            is_locked_out = until and now < until

            return is_locked_out
        except ObjectDoesNotExist:
            return False

    @classmethod
    def increment_lockout_counter(cls, user):
        """
        Ticks the failed attempt counter
        """
        record, _ = LoginFailures.objects.get_or_create(user=user)
        record.failure_count = record.failure_count + 1
        max_failures_allowed = settings.MAX_FAILED_LOGIN_ATTEMPTS_ALLOWED

        # did we go over the limit in attempts
        if record.failure_count >= max_failures_allowed:
            # yes, then store when this account is locked out until
            lockout_period_secs = settings.MAX_FAILED_LOGIN_ATTEMPTS_LOCKOUT_PERIOD_SECS
            record.lockout_until = datetime.now(UTC) + timedelta(seconds=lockout_period_secs)

        record.save()

    @classmethod
    def clear_lockout_counter(cls, user):
        """
        Removes the lockout counters (normally called after a successful login)
        """
        try:
            entry = cls._get_record_for_user(user)
            entry.delete()
        except ObjectDoesNotExist:
            return

    def __str__(self):
        """Str -> Username: count - date."""
        return u'{username}: {count} - {date}'.format(
            username=self.user.username,
            count=self.failure_count,
            date=self.lockout_until.isoformat() if self.lockout_until else '-'
        )

    class Meta:
        verbose_name = 'Login Failure'
        verbose_name_plural = 'Login Failures'


class CourseEnrollmentException(Exception):
    pass


class NonExistentCourseError(CourseEnrollmentException):
    pass


class EnrollmentClosedError(CourseEnrollmentException):
    pass


class CourseFullError(CourseEnrollmentException):
    pass


class AlreadyEnrolledError(CourseEnrollmentException):
    pass


class CourseEnrollmentManager(models.Manager):
    """
    Custom manager for CourseEnrollment with Table-level filter methods.
    """

    def num_enrolled_in(self, course_id):
        """
        Returns the count of active enrollments in a course.

        'course_id' is the course_id to return enrollments
        """

        enrollment_number = super(CourseEnrollmentManager, self).get_queryset().filter(
            course_id=course_id,
            is_active=1
        ).count()

        return enrollment_number

    def num_enrolled_in_exclude_admins(self, course_id):
        """
        Returns the count of active enrollments in a course excluding instructors, staff and CCX coaches.

        Arguments:
            course_id (CourseLocator): course_id to return enrollments (count).

        Returns:
            int: Count of enrollments excluding staff, instructors and CCX coaches.

        """
        # To avoid circular imports.
        from student.roles import CourseCcxCoachRole, CourseInstructorRole, CourseStaffRole
        course_locator = course_id

        if getattr(course_id, 'ccx', None):
            course_locator = course_id.to_course_locator()

        staff = CourseStaffRole(course_locator).users_with_role()
        admins = CourseInstructorRole(course_locator).users_with_role()
        coaches = CourseCcxCoachRole(course_locator).users_with_role()