# -*- coding: utf-8 -*-
"""
Certificates are created for a student and an offering of a course.

When a certificate is generated, a unique ID is generated so that
the certificate can be verified later. The ID is a UUID4, so that
it can't be easily guessed and so that it is unique.

Certificates are generated in batches by a cron job, when a
certificate is available for download the GeneratedCertificate
table is updated with information that will be displayed
on the course overview page.


State diagram:

[deleted,error,unavailable] [error,downloadable]
            +                +             +
            |                |             |
            |                |             |
         add_cert       regen_cert     del_cert
            |                |             |
            v                v             v
       [generating]    [regenerating]  [deleting]
            +                +             +
            |                |             |
       certificate      certificate    certificate
         created       removed,created   deleted
            +----------------+-------------+------->[error]
            |                |             |
            |                |             |
            v                v             v
      [downloadable]   [downloadable]  [deleted]


Eligibility:

    Students are eligible for a certificate if they pass the course
    with the following exceptions:

       If the student has allow_certificate set to False in the student profile
       he will never be issued a certificate.

       If the user and course is present in the certificate whitelist table
       then the student will be issued a certificate regardless of his grade,
       unless he has allow_certificate set to False.
"""
import json
import logging
import os
import uuid

from config_models.models import ConfigurationModel
from django.conf import settings
from django.contrib.auth.models import User
from django.core.exceptions import ValidationError
from django.db import models, transaction
from django.db.models import Count
from django.dispatch import receiver
from django.utils.translation import ugettext_lazy as _
from django_extensions.db.fields import CreationDateTimeField
from model_utils import Choices
from model_utils.models import TimeStampedModel

from badges.events.course_complete import course_badge_check
from badges.events.course_meta import completion_check, course_group_check
from lms.djangoapps.instructor_task.models import InstructorTask
from openedx.core.djangoapps.signals.signals import COURSE_CERT_AWARDED
from openedx.core.djangoapps.xmodule_django.models import CourseKeyField, NoneToEmptyManager
from util.milestones_helpers import fulfill_course_milestone, is_prerequisite_courses_enabled

LOGGER = logging.getLogger(__name__)


class CertificateStatuses(object):
    """
    Enum for certificate statuses
    """
    deleted = 'deleted'
    deleting = 'deleting'
    downloadable = 'downloadable'
    error = 'error'
    generating = 'generating'
    notpassing = 'notpassing'
    restricted = 'restricted'
    unavailable = 'unavailable'
    auditing = 'auditing'
    audit_passing = 'audit_passing'
    audit_notpassing = 'audit_notpassing'
    unverified = 'unverified'
    invalidated = 'invalidated'
    requesting = 'requesting'

    readable_statuses = {
        downloadable: "already received",
        notpassing: "didn't receive",
        error: "error states",
        audit_passing: "audit passing states",
        audit_notpassing: "audit not passing states",
    }

    PASSED_STATUSES = (downloadable, generating)

    @classmethod
    def is_passing_status(cls, status):
        """
        Given the status of a certificate, return a boolean indicating whether
        the student passed the course.
        """
        return status in cls.PASSED_STATUSES


class CertificateSocialNetworks(object):
    """
    Enum for certificate social networks
    """
    linkedin = 'LinkedIn'
    facebook = 'Facebook'
    twitter = 'Twitter'


class CertificateWhitelist(models.Model):
    """
    Tracks students who are whitelisted, all users
    in this table will always qualify for a certificate
    regardless of their grade unless they are on the
    embargoed country restriction list
    (allow_certificate set to False in userprofile).
    """
    class Meta(object):
        app_label = "certificates"

    objects = NoneToEmptyManager()

    user = models.ForeignKey(User)
    course_id = CourseKeyField(max_length=255, blank=True, default=None)
    whitelist = models.BooleanField(default=0)
    created = CreationDateTimeField(_('created'))
    notes = models.TextField(default=None, null=True)

    @classmethod
    def get_certificate_white_list(cls, course_id, student=None):
        """
        Return certificate white list for the given course as dict object,
        returned dictionary will have the following key-value pairs

        [{
            id:         'id (pk) of CertificateWhitelist item'
            user_id:    'User Id of the student'
            user_name:  'name of the student'
            user_email: 'email of the student'
            course_id:  'Course key of the course to whom certificate exception belongs'
            created:    'Creation date of the certificate exception'
            notes:      'Additional notes for the certificate exception'
        }, {...}, ...]

        """
        white_list = cls.objects.filter(course_id=course_id, whitelist=True)
        if student:
            white_list = white_list.filter(user=student)
        result = []
        generated_certificates = GeneratedCertificate.eligible_certificates.filter(
            course_id=course_id,
            user__in=[exception.user for exception in white_list],
            status=CertificateStatuses.downloadable
        )
        generated_certificates = {
            certificate['user']: certificate['created_date']
            for certificate in generated_certificates.values('user', 'created_date')
        }

        for item in white_list:
            certificate_generated = generated_certificates.get(item.user.id, '')
            result.append({
                'id': item.id,
                'user_id': item.user.id,
                'user_name': unicode(item.user.username),
                'user_email': unicode(item.user.email),
                'course_id': unicode(item.course_id),
                'created': item.created.strftime("%B %d, %Y"),
                'certificate_generated': certificate_generated and certificate_generated.strftime("%B %d, %Y"),
                'notes': unicode(item.notes or ''),
            })
        return result


class EligibleCertificateManager(models.Manager):
    """
    A manager for `GeneratedCertificate` models that automatically
    filters out ineligible certs.

    The idea is to prevent accidentally granting certificates to
    students who have not enrolled in a cert-granting mode. The
    alternative is to filter by eligible_for_certificate=True every
    time certs are searched for, which is verbose and likely to be
    forgotten.
    """

    def get_queryset(self):
        """
        Return a queryset for `GeneratedCertificate` models, filtering out
        ineligible certificates.
        """
        return super(EligibleCertificateManager, self).get_queryset().exclude(
            status__in=(CertificateStatuses.audit_passing, CertificateStatuses.audit_notpassing)
        )


class GeneratedCertificate(models.Model):
    """
    Base model for generated certificates
    """
    # Import here instead of top of file since this module gets imported before
    # the course_modes app is loaded, resulting in a Django deprecation warning.
    from course_modes.models import CourseMode

    # Only returns eligible certificates. This should be used in
    # preference to the default `objects` manager in most cases.
    eligible_certificates = EligibleCertificateManager()

    # Normal object manager, which should only be used when ineligible
    # certificates (i.e. new audit certs) should be included in the
    # results. Django requires us to explicitly declare this.
    objects = models.Manager()

    MODES = Choices('verified', 'honor', 'audit', 'professional', 'no-id-professional')

    VERIFIED_CERTS_MODES = [CourseMode.VERIFIED, CourseMode.CREDIT_MODE]

    user = models.ForeignKey(User)
    course_id = CourseKeyField(max_length=255, blank=True, default=None)
    verify_uuid = models.CharField(max_length=32, blank=True, default='', db_index=True)
    download_uuid = models.CharField(max_length=32, blank=True, default='')
    download_url = models.CharField(max_length=128, blank=True, default='')
    grade = models.CharField(max_length=5, blank=True, default='')
    key = models.CharField(max_length=32, blank=True, default='')
    distinction = models.BooleanField(default=False)
    status = models.CharField(max_length=32, default='unavailable')
    mode = models.CharField(max_length=32, choices=MODES, default=MODES.honor)
    name = models.CharField(blank=True, max_length=255)
    created_date = models.DateTimeField(auto_now_add=True)
    modified_date = models.DateTimeField(auto_now=True)
    error_reason = models.CharField(max_length=512, blank=True, default='')

    class Meta(object):
        unique_together = (('user', 'course_id'),)
        app_label = "certificates"

    @classmethod
    def certificate_for_student(cls, student, course_id):
        """
        This returns the certificate for a student for a particular course
        or None if no such certificate exits.
        """
        try:
            return cls.objects.get(user=student, course_id=course_id)
        except cls.DoesNotExist:
            pass

        return None

    @classmethod
    def course_ids_with_certs_for_user(cls, user):
        """
        Return a set of CourseKeys for which the user has certificates.

        Sometimes we just want to check if a user has already been issued a
        certificate for a given course (e.g. to test refund elibigility).
        Instead of checking if `certificate_for_student` returns `None` on each
        course_id individually, we instead just return a set of all CourseKeys
        for which this student has certificates all at once.
        """
        return {
            cert.course_id
            for cert
            in cls.objects.filter(user=user).only('course_id')  # pylint: disable=no-member
        }

    @classmethod
    def get_unique_statuses(cls, course_key=None, flat=False):
        """
        1 - Return unique statuses as a list of dictionaries containing the following key value pairs
            [
            {'status': 'status value from db', 'count': 'occurrence count of the status'},
            {...},
            ..., ]

        2 - if flat is 'True' then return unique statuses as a list
        3 - if course_key is given then return unique statuses associated with the given course

        :param course_key: Course Key identifier
        :param flat: boolean showing whether to return statuses as a list of values or a list of dictionaries.
        """
        query = cls.objects

        if course_key:
            query = query.filter(course_id=course_key)

        if flat:
            return query.values_list('status', flat=True).distinct()
        else:
            return query.values('status').annotate(count=Count('status'))

    def __repr__(self):
        return "<GeneratedCertificate: {course_id}, user={user}>".format(
            course_id=self.course_id,
            user=self.user
        )

    def invalidate(self):
        """
        Invalidate Generated Certificate by  marking it 'unavailable'.

        Following is the list of fields with their defaults
            1 - verify_uuid = '',
            2 - download_uuid = '',
            3 - download_url = '',
            4 - grade = ''
            5 - status = 'unavailable'
        """
        self.verify_uuid = ''
        self.download_uuid = ''
        self.download_url = ''
        self.grade = ''
        self.status = CertificateStatuses.unavailable

        self.save()

    def is_valid(self):
        """
        Return True if certificate is valid else return False.
        """
        return self.status == CertificateStatuses.downloadable

    def save(self, *args, **kwargs):
        """
        After the base save() method finishes, fire the COURSE_CERT_AWARDED
        signal iff we are saving a record of a learner passing the course.
        """
        super(GeneratedCertificate, self).save(*args, **kwargs)
        if CertificateStatuses.is_passing_status(self.status):
            COURSE_CERT_AWARDED.send_robust(
                sender=self.__class__,
                user=self.user,
                course_key=self.course_id,
                mode=self.mode,
                status=self.status,
            )


class CertificateGenerationHistory(TimeStampedModel):
    """
    Model for storing Certificate Generation History.
    """

    course_id = CourseKeyField(max_length=255)
    generated_by = models.ForeignKey(User)
    instructor_task = models.ForeignKey(InstructorTask)
    is_regeneration = models.BooleanField(default=False)

    def get_task_name(self):
        """
        Return "regenerated" if record corresponds to Certificate Regeneration task, otherwise returns 'generated'
        """
        # Translators: This is a past-tense verb that is used for task action messages.
        return _("regenerated") if self.is_regeneration else _("generated")

    def get_certificate_generation_candidates(self):
        """
        Return the candidates for certificate generation task. It could either be students or certificate statuses
        depending upon the nature of certificate generation task. Returned value could be one of the following,

        1. "All learners" Certificate Generation task was initiated for all learners of the given course.
        2. Comma separated list of certificate statuses, This usually happens when instructor regenerates certificates.
        3. "for exceptions", This is the case when instructor generates certificates for white-listed
            students.
        """
        task_input = self.instructor_task.task_input
        if not task_input.strip():
            # if task input is empty, it means certificates were generated for all learners
            # Translators: This string represents task was executed for all learners.
            return _("All learners")

        task_input_json = json.loads(task_input)

        # get statuses_to_regenerate from task_input convert statuses to human readable strings and return
        statuses = task_input_json.get('statuses_to_regenerate', None)
        if statuses:
            readable_statuses = [
                CertificateStatuses.readable_statuses.get(status) for status in statuses
                if CertificateStatuses.readable_statuses.get(status) is not None
            ]
            return ", ".join(readable_statuses)

        # If "student_set" is present in task_input, then this task only
        # generates certificates for white listed students. Note that
        # this key used to be "students", so we include that in this conditional
        # for backwards compatibility.
        if 'student_set' in task_input_json or 'students' in task_input_json:
            # Translators: This string represents task was executed for students having exceptions.
            return _("For exceptions")
        else:
            return _("All learners")

    class Meta(object):
        app_label = "certificates"

    def __unicode__(self):
        return u"certificates %s by %s on %s for %s" % \
               ("regenerated" if self.is_regeneration else "generated", self.generated_by, self.created, self.course_id)


class CertificateInvalidation(TimeStampedModel):
    """
    Model for storing Certificate Invalidation.
    """
    generated_certificate = models.ForeignKey(GeneratedCertificate)
    invalidated_by = models.ForeignKey(User)
    notes = models.TextField(default=None, null=True)
    active = models.BooleanField(default=True)

    class Meta(object):
        app_label = "certificates"

    def __unicode__(self):
        return u"Certificate %s, invalidated by %s on %s." % \
               (self.generated_certificate, self.invalidated_by, self.created)

    def deactivate(self):
        """
        Deactivate certificate invalidation by setting active to False.
        """
        self.active = False
        self.save()

    @classmethod
    def get_certificate_invalidations(cls, course_key, student=None):
        """
        Return certificate invalidations filtered based on the provided course and student (if provided),

        Returned value is JSON serializable list of dicts, dict element would have the following key-value pairs.
         1. id: certificate invalidation id (primary key)
         2. user: username of the student to whom certificate belongs
         3. invalidated_by: user id of the instructor/support user who invalidated the certificate
         4. created: string containing date of invalidation in the following format "December 29, 2015"
         5. notes: string containing notes regarding certificate invalidation.
        """
        certificate_invalidations = cls.objects.filter(
            generated_certificate__course_id=course_key,
            active=True,
        )
        if student:
            certificate_invalidations = certificate_invalidations.filter(generated_certificate__user=student)
        data = []
        for certificate_invalidation in certificate_invalidations:
            data.append({
                'id': certificate_invalidation.id,
                'user': certificate_invalidation.generated_certificate.user.username,
                'invalidated_by': certificate_invalidation.invalidated_by.username,
                'created': certificate_invalidation.created.strftime("%B %d, %Y"),
                'notes': certificate_invalidation.notes,
            })
        return data

    @classmethod
    def has_certificate_invalidation(cls, student, course_key):
        """Check that whether the student in the course has been invalidated
        for receiving certificates.

        Arguments:
            student (user): logged-in user
            course_key (CourseKey): The course associated with the certificate.

        Returns:
             Boolean denoting whether the student in the course is invalidated
             to receive certificates
        """
        return cls.objects.filter(
            generated_certificate__course_id=course_key,
            active=True,
            generated_certificate__user=student
        ).exists()


@receiver(COURSE_CERT_AWARDED, sender=GeneratedCertificate)
def handle_course_cert_awarded(sender, user, course_key, **kwargs):  # pylint: disable=unused-argument
    """
    Mark a milestone entry if user has passed the course.
    """
    if is_prerequisite_courses_enabled():
        fulfill_course_milestone(course_key, user)


def certificate_status_for_student(student, course_id):
    """
    This returns a dictionary with a key for status, and other information.
    See certificate_status for more information.
    """
    try:
        generated_certificate = GeneratedCertificate.objects.get(user=student, course_id=course_id)
    except GeneratedCertificate.DoesNotExist:
        generated_certificate = None
    return certificate_status(generated_certificate)


def certificate_status(generated_certificate):
    '''
    This returns a dictionary with a key for status, and other information.
    The status is one of the following:

    unavailable  - No entry for this student--if they are actually in
                   the course, they probably have not been graded for
                   certificate generation yet.
    generating   - A request has been made to generate a certificate,
                   but it has not been generated yet.
    regenerating - A request has been made to regenerate a certificate,
                   but it has not been generated yet.
    deleting     - A request has been made to delete a certificate.

    deleted      - The certificate has been deleted.
    downloadable - The certificate is available for download.
    notpassing   - The student was graded but is not passing
    restricted   - The student is on the restricted embargo list and
                   should not be issued a certificate. This will
                   be set if allow_certificate is set to False in
                   the userprofile table
    unverified   - The student is in verified enrollment track and
                   the student did not have their identity verified,
                   even though they should be eligible for the cert otherwise.

    If the status is "downloadable", the dictionary also contains
    "download_url".

    If the student has been graded, the dictionary also contains their
    grade for the course with the key "grade".
    '''
    # Import here instead of top of file since this module gets imported before
    # the course_modes app is loaded, resulting in a Django deprecation warning.
    from course_modes.models import CourseMode

    if generated_certificate:
        cert_status = {
            'status': generated_certificate.status,
            'mode': generated_certificate.mode,
            'uuid': generated_certificate.verify_uuid,
        }
        if generated_certificate.grade:
            cert_status['grade'] = generated_certificate.grade

        if generated_certificate.mode == 'audit':
            course_mode_slugs = [mode.slug for mode in CourseMode.modes_for_course(generated_certificate.course_id)]
            # Short term fix to make sure old audit users with certs still see their certs
            # only do this if there if no honor mode
            if 'honor' not in course_mode_slugs:
                cert_status['status'] = CertificateStatuses.auditing
                return cert_status

        if generated_certificate.status == CertificateStatuses.downloadable:
            cert_status['download_url'] = generated_certificate.download_url

        return cert_status
    else:
        return {'status': CertificateStatuses.unavailable, 'mode': GeneratedCertificate.MODES.honor, 'uuid': None}


def certificate_info_for_user(user, grade, user_is_whitelisted, user_certificate):
    """
    Returns the certificate info for a user for grade report.
    """
    certificate_is_delivered = 'N'
    certificate_type = 'N/A'
    eligible_for_certificate = 'Y' if (user_is_whitelisted or grade is not None) and user.profile.allow_certificate \
        else 'N'

    status = certificate_status(user_certificate)
    certificate_generated = status['status'] == CertificateStatuses.downloadable
    if certificate_generated:
        certificate_is_delivered = 'Y'
        certificate_type = status['mode']

    return [eligible_for_certificate, certificate_is_delivered, certificate_type]


class ExampleCertificateSet(TimeStampedModel):
    """A set of example certificates.

    Example certificates are used to verify that certificate
    generation is working for a particular course.

    A particular course may have several kinds of certificates
    (e.g. honor and verified), in which case we generate
    multiple example certificates for the course.

    """
    course_key = CourseKeyField(max_length=255, db_index=True)

    class Meta(object):
        get_latest_by = 'created'
        app_label = "certificates"

    @classmethod
    @transaction.atomic
    def create_example_set(cls, course_key):
        """Create a set of example certificates for a course.

        Arguments:
            course_key (CourseKey)

        Returns:
            ExampleCertificateSet

        """
        # Import here instead of top of file since this module gets imported before
        # the course_modes app is loaded, resulting in a Django deprecation warning.
        from course_modes.models import CourseMode
        cert_set = cls.objects.create(course_key=course_key)

        ExampleCertificate.objects.bulk_create([
            ExampleCertificate(
                example_cert_set=cert_set,
                description=mode.slug,
                template=cls._template_for_mode(mode.slug, course_key)
            )
            for mode in CourseMode.modes_for_course(course_key)
        ])

        return cert_set

    @classmethod
    def latest_status(cls, course_key):
        """Summarize the latest status of example certificates for a course.

        Arguments:
            course_key (CourseKey)

        Returns:
            list: List of status dictionaries.  If no example certificates
                have been started yet, returns None.

        """
        try:
            latest = cls.objects.filter(course_key=course_key).latest()
        except cls.DoesNotExist:
            return None

        queryset = ExampleCertificate.objects.filter(example_cert_set=latest).order_by('-created')
        return [cert.status_dict for cert in queryset]

    def __iter__(self):
        """Iterate through example certificates in the set.

        Yields:
            ExampleCertificate

        """
        queryset = (ExampleCertificate.objects).select_related('example_cert_set').filter(example_cert_set=self)
        for cert in queryset:
            yield cert

    @staticmethod
    def _template_for_mode(mode_slug, course_key):
        """Calculate the template PDF based on the course mode. """
        return (
            u"certificate-template-{key.org}-{key.course}-verified.pdf".format(key=course_key)
            if mode_slug == 'verified'
            else u"certificate-template-{key.org}-{key.course}.pdf".format(key=course_key)
        )


def _make_uuid():
    """Return a 32-character UUID. """
    return uuid.uuid4().hex


class ExampleCertificate(TimeStampedModel):
    """Example certificate.

    Example certificates are used to verify that certificate
    generation is working for a particular course.

    An example certificate is similar to an ordinary certificate,
    except that:

    1) Example certificates are not associated with a particular user,
        and are never displayed to students.

    2) We store the "inputs" for generating the example certificate
        to make it easier to debug when certificate generation fails.

    3) We use dummy values.

    """
    class Meta(object):
        app_label = "certificates"

    # Statuses
    STATUS_STARTED = 'started'
    STATUS_SUCCESS = 'success'
    STATUS_ERROR = 'error'

    # Dummy full name for the generated certificate
    EXAMPLE_FULL_NAME = u'John Doë'

    example_cert_set = models.ForeignKey(ExampleCertificateSet)

    description = models.CharField(
        max_length=255,
        help_text=_(
            u"A human-readable description of the example certificate.  "
            u"For example, 'verified' or 'honor' to differentiate between "
            u"two types of certificates."
        )
    )

    # Inputs to certificate generation
    # We store this for auditing purposes if certificate
    # generation fails.
    uuid = models.CharField(
        max_length=255,
        default=_make_uuid,
        db_index=True,
        unique=True,
        help_text=_(
            u"A unique identifier for the example certificate.  "
            u"This is used when we receive a response from the queue "
            u"to determine which example certificate was processed."
        )
    )

    access_key = models.CharField(
        max_length=255,
        default=_make_uuid,
        db_index=True,
        help_text=_(
            u"An access key for the example certificate.  "
            u"This is used when we receive a response from the queue "
            u"to validate that the sender is the same entity we asked "
            u"to generate the certificate."
        )
    )

    full_name = models.CharField(
        max_length=255,
        default=EXAMPLE_FULL_NAME,
        help_text=_(u"The full name that will appear on the certificate.")
    )

    template = models.CharField(
        max_length=255,
        help_text=_(u"The template file to use when generating the certificate.")
    )

    # Outputs from certificate generation
    status = models.CharField(
        max_length=255,
        default=STATUS_STARTED,
        choices=(
            (STATUS_STARTED, 'Started'),
            (STATUS_SUCCESS, 'Success'),
            (STATUS_ERROR, 'Error')
        ),
        help_text=_(u"The status of the example certificate.")
    )

    error_reason = models.TextField(
        null=True,
        default=None,
        help_text=_(u"The reason an error occurred during certificate generation.")
    )

    download_url = models.CharField(
        max_length=255,
        null=True,
        default=None,
        help_text=_(u"The download URL for the generated certificate.")
    )

    def update_status(self, status, error_reason=None, download_url=None):
        """Update the status of the example certificate.

        This will usually be called either:
        1) When an error occurs adding the certificate to the queue.
        2) When we receieve a response from the queue (either error or success).

        If an error occurs, we store the error message;
        if certificate generation is successful, we store the URL
        for the generated certificate.

        Arguments:
            status (str): Either `STATUS_SUCCESS` or `STATUS_ERROR`

        Keyword Arguments:
            error_reason (unicode): A description of the error that occurred.
            download_url (unicode): The URL for the generated certificate.

        Raises:
            ValueError: The status is not a valid value.

        """
        if status not in [self.STATUS_SUCCESS, self.STATUS_ERROR]:
            msg = u"Invalid status: must be either '{success}' or '{error}'.".format(
                success=self.STATUS_SUCCESS,
                error=self.STATUS_ERROR
            )
            raise ValueError(msg)

        self.status = status

        if status == self.STATUS_ERROR and error_reason:
            self.error_reason = error_reason

        if status == self.STATUS_SUCCESS and download_url:
            self.download_url = download_url

        self.save()

    @property
    def status_dict(self):
        """Summarize the status of the example certificate.

        Returns:
            dict

        """
        result = {
            'description': self.description,
            'status': self.status,
        }

        if self.error_reason:
            result['error_reason'] = self.error_reason

        if self.download_url:
            result['download_url'] = self.download_url

        return result

    @property
    def course_key(self):
        """The course key associated with the example certificate. """
        return self.example_cert_set.course_key


class CertificateGenerationCourseSetting(TimeStampedModel):
    """Enable or disable certificate generation for a particular course.

    This controls whether students are allowed to "self-generate"
    certificates for a course.  It does NOT prevent us from
    batch-generating certificates for a course using management
    commands.

    In general, we should only enable self-generated certificates
    for a course once we successfully generate example certificates
    for the course.  This is enforced in the UI layer, but
    not in the data layer.

    """
    course_key = CourseKeyField(max_length=255, db_index=True)
    enabled = models.BooleanField(default=False)

    class Meta(object):
        get_latest_by = 'created'
        app_label = "certificates"

    @classmethod
    def is_enabled_for_course(cls, course_key):
        """Check whether self-generated certificates are enabled for a course.

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

        Returns:
            boolean

        """
        try:
            latest = cls.objects.filter(course_key=course_key).latest()
        except cls.DoesNotExist:
            return False
        else:
            return latest.enabled

    @classmethod
    def set_enabled_for_course(cls, course_key, is_enabled):
        """Enable or disable self-generated certificates for a course.

        Arguments:
            course_key (CourseKey): The identifier for the course.
            is_enabled (boolean): Whether to enable or disable self-generated certificates.

        """
        CertificateGenerationCourseSetting.objects.create(
            course_key=course_key,
            enabled=is_enabled
        )


class CertificateGenerationConfiguration(ConfigurationModel):
    """Configure certificate generation.

    Enable or disable the self-generated certificates feature.
    When this flag is disabled, the "generate certificate" button
    will be hidden on the progress page.

    When the feature is enabled, the "generate certificate" button
    will appear for courses that have enabled self-generated
    certificates.

    """
    class Meta(ConfigurationModel.Meta):
        app_label = "certificates"


class CertificateHtmlViewConfiguration(ConfigurationModel):
    """
    Static values for certificate HTML view context parameters.
    Default values will be applied across all certificate types (course modes)
    Matching 'mode' overrides will be used instead of defaults, where applicable
    Example configuration :
        {
            "default": {
                "url": "http://www.edx.org",
                "logo_src": "http://www.edx.org/static/images/logo.png"
            },
            "honor": {
                "logo_src": "http://www.edx.org/static/images/honor-logo.png"
            }
        }
    """
    class Meta(ConfigurationModel.Meta):
        app_label = "certificates"

    configuration = models.TextField(
        help_text="Certificate HTML View Parameters (JSON)"
    )

    def clean(self):
        """
        Ensures configuration field contains valid JSON.
        """
        try:
            json.loads(self.configuration)
        except ValueError:
            raise ValidationError('Must be valid JSON string.')

    @classmethod
    def get_config(cls):
        """
        Retrieves the configuration field value from the database
        """
        instance = cls.current()
        json_data = json.loads(instance.configuration) if instance.enabled else {}
        return json_data


class CertificateTemplate(TimeStampedModel):
    """A set of custom web certificate templates.

    Web certificate templates are Django web templates
    to replace PDF certificate.

    A particular course may have several kinds of certificate templates
    (e.g. honor and verified).

    """
    name = models.CharField(
        max_length=255,
        help_text=_(u'Name of template.'),
    )
    description = models.CharField(
        max_length=255,
        null=True,
        blank=True,
        help_text=_(u'Description and/or admin notes.'),
    )
    template = models.TextField(
        help_text=_(u'Django template HTML.'),
    )
    organization_id = models.IntegerField(
        null=True,
        blank=True,
        db_index=True,
        help_text=_(u'Organization of template.'),
    )
    course_key = CourseKeyField(
        max_length=255,
        null=True,
        blank=True,
        db_index=True,
    )
    mode = models.CharField(
        max_length=125,
        choices=GeneratedCertificate.MODES,
        default=GeneratedCertificate.MODES.honor,
        null=True,
        blank=True,
        help_text=_(u'The course mode for this template.'),
    )
    is_active = models.BooleanField(
        help_text=_(u'On/Off switch.'),
        default=False,
    )

    def __unicode__(self):
        return u'%s' % (self.name, )

    class Meta(object):
        get_latest_by = 'created'
        unique_together = (('organization_id', 'course_key', 'mode'),)
        app_label = "certificates"


def template_assets_path(instance, filename):
    """
    Delete the file if it already exist and returns the certificate template asset file path.

    :param instance: CertificateTemplateAsset object
    :param filename: file to upload
    :return path: path of asset file e.g. certificate_template_assets/1/filename
    """
    name = os.path.join('certificate_template_assets', str(instance.id), filename)
    fullname = os.path.join(settings.MEDIA_ROOT, name)
    if os.path.exists(fullname):
        os.remove(fullname)
    return name


class CertificateTemplateAsset(TimeStampedModel):
    """A set of assets to be used in custom web certificate templates.

    This model stores assets used in custom web certificate templates
    such as image, css files.

    """
    description = models.CharField(
        max_length=255,
        null=True,
        blank=True,
        help_text=_(u'Description of the asset.'),
    )
    asset = models.FileField(
        max_length=255,
        upload_to=template_assets_path,
        help_text=_(u'Asset file. It could be an image or css file.'),
    )
    asset_slug = models.SlugField(
        max_length=255,
        unique=True,
        null=True,
        help_text=_(u'Asset\'s unique slug. We can reference the asset in templates using this value.'),
    )

    def save(self, *args, **kwargs):
        """save the certificate template asset """
        if self.pk is None:
            asset_image = self.asset
            self.asset = None
            super(CertificateTemplateAsset, self).save(*args, **kwargs)
            self.asset = asset_image

        super(CertificateTemplateAsset, self).save(*args, **kwargs)

    def __unicode__(self):
        return u'%s' % (self.asset.url, )

    class Meta(object):
        get_latest_by = 'created'
        app_label = "certificates"


@receiver(COURSE_CERT_AWARDED, sender=GeneratedCertificate)
# pylint: disable=unused-argument
def create_course_badge(sender, user, course_key, status, **kwargs):
    """
    Standard signal hook to create course badges when a certificate has been generated.
    """
    course_badge_check(user, course_key)


@receiver(COURSE_CERT_AWARDED, sender=GeneratedCertificate)
def create_completion_badge(sender, user, course_key, status, **kwargs):  # pylint: disable=unused-argument
    """
    Standard signal hook to create 'x courses completed' badges when a certificate has been generated.
    """
    completion_check(user)


@receiver(COURSE_CERT_AWARDED, sender=GeneratedCertificate)
def create_course_group_badge(sender, user, course_key, status, **kwargs):  # pylint: disable=unused-argument
    """
    Standard signal hook to create badges when a user has completed a prespecified set of courses.
    """
    course_group_check(user, course_key)
