models.py

    visit = models.ForeignKey(FlowPageVisit, related_name="grades",
            verbose_name=_('Visit'), on_delete=models.CASCADE)

    # NULL means 'autograded'
    grader = models.ForeignKey(settings.AUTH_USER_MODEL, null=True, blank=True,
            verbose_name=_('Grader'), on_delete=models.SET_NULL)
    grade_time = models.DateTimeField(db_index=True, default=now,
            verbose_name=_('Grade time'))

    graded_at_git_commit_sha = models.CharField(
            max_length=200, null=True, blank=True,
            verbose_name=_('Graded at git commit SHA'))

    grade_data = JSONField(null=True, blank=True,
            # Show correct characters in admin for non ascii languages.
            dump_kwargs={'ensure_ascii': False},
            verbose_name=_('Grade data'))

    # This data should be recomputable, but we'll cache it here,
    # because it might be very expensive (container-launch expensive
    # for code questions, for example) to recompute.

    max_points = models.FloatField(null=True, blank=True,
            # Translators: max point in grade
            help_text=_("Point value of this question when receiving "
            "full credit."),
            verbose_name=_('Max points'))
    correctness = models.FloatField(null=True, blank=True,
            # Translators: correctness in grade
            help_text=_("Real number between zero and one (inclusively) "
            "indicating the degree of correctness of the answer."),
            verbose_name=_('Correctness'))

    # This JSON object has fields corresponding to
    # :class:`course.page.AnswerFeedback`, except for
    # :attr:`course.page.AnswerFeedback.correctness`, which is stored
    # separately for efficiency.

    feedback = JSONField(null=True, blank=True,
            # Show correct characters in admin for non ascii languages.
            dump_kwargs={'ensure_ascii': False},
            # Translators: "Feedback" stands for the feedback of answers.
            verbose_name=_('Feedback'))

    def percentage(self):
        if self.correctness is not None:
            return 100*self.correctness
        else:
            return None

    def value(self):
        if self.correctness is not None and self.max_points is not None:
            return self.correctness * self.max_points
        else:
            return None

    class Meta:
        verbose_name = _("Flow page visit grade")
        verbose_name_plural = _("Flow page visit grades")
        # These must be distinguishable, to figure out what came later.
        unique_together = (("visit", "grade_time"),)

        ordering = ("visit", "grade_time")

    def __unicode__(self):
        # information on FlowPageVisitGrade class
        # Translators: return the information of the grade of a user
        # by percentage.
        return _("grade of %(visit)s: %(percentage)s") % {
                "visit": self.visit, "percentage": self.percentage()}

    if six.PY3:
        __str__ = __unicode__


class FlowPageBulkFeedback(models.Model):
    # We're only storing one of these per page, because
    # they're 'bulk' (i.e. big, like plots or program output)
    page_data = models.OneToOneField(FlowPageData,
            verbose_name=_('Page data'), on_delete=models.CASCADE)
    grade = models.ForeignKey(FlowPageVisitGrade,
            verbose_name=_('Grade'), on_delete=models.CASCADE)

    bulk_feedback = JSONField(null=True, blank=True,
            # Show correct characters in admin for non ascii languages.
            dump_kwargs={'ensure_ascii': False},
            verbose_name=_('Bulk feedback'))


def update_bulk_feedback(page_data, grade, bulk_feedback_json):
    # type: (FlowPageData, FlowPageVisitGrade, Any) -> None
    FlowPageBulkFeedback.objects.update_or_create(
            page_data=page_data,
            defaults=dict(
                grade=grade,
                bulk_feedback=bulk_feedback_json))


def get_feedback_for_grade(grade):
    # type: (FlowPageVisitGrade) -> Optional[AnswerFeedback]

    try:
        bulk_feedback_json = FlowPageBulkFeedback.objects.get(
                page_data=grade.visit.page_data,
                grade=grade).bulk_feedback
    except ObjectDoesNotExist:
        bulk_feedback_json = None

    if grade is not None:
        return AnswerFeedback.from_json(
                grade.feedback, bulk_feedback_json)
    else:
        return None

# }}}


# {{{ flow access

def validate_stipulations(stip):
    if stip is None:
        return

    if not isinstance(stip, dict):
        raise ValidationError(_("stipulations must be a dictionary"))
    allowed_keys = set(["credit_percent", "allowed_session_count"])
    if not set(stip.keys()) <= allowed_keys:
        raise ValidationError(
                string_concat(
                    _("unrecognized keys in stipulations"),
                    ": %s")
                % ", ".join(set(stip.keys()) - allowed_keys))

    if "credit_percent" in stip and not isinstance(
            stip["credit_percent"], (int, float)):
        raise ValidationError(_("credit_percent must be a float"))
    if ("allowed_session_count" in stip
            and (
                not isinstance(stip["allowed_session_count"], int)
                or stip["allowed_session_count"] < 0)):
        raise ValidationError(
                _("'allowed_session_count' must be a non-negative integer"))


# {{{ deprecated exception stuff

class FlowAccessException(models.Model):
    # deprecated

    participation = models.ForeignKey(Participation, db_index=True,
            verbose_name=_('Participation'), on_delete=models.CASCADE)
    flow_id = models.CharField(max_length=200, blank=False, null=False,
            verbose_name=_('Flow ID'))
    expiration = models.DateTimeField(blank=True, null=True,
            verbose_name=_('Expiration'))

    stipulations = JSONField(blank=True, null=True,
            # Translators: help text for stipulations in FlowAccessException
            # (deprecated)
            help_text=_("A dictionary of the same things that can be added "
            "to a flow access rule, such as allowed_session_count or "
            "credit_percent. If not specified here, values will default "
            "to the stipulations in the course content."),
            validators=[validate_stipulations],
            dump_kwargs={'ensure_ascii': False},
            verbose_name=_('Stipulations'))

    creator = models.ForeignKey(settings.AUTH_USER_MODEL, null=True,
            verbose_name=_('Creator'), on_delete=models.SET_NULL)
    creation_time = models.DateTimeField(default=now, db_index=True,
            verbose_name=_('Creation time'))

    is_sticky = models.BooleanField(
            default=False,
            # Translators: deprecated
            help_text=_("Check if a flow started under this "
            "exception rule set should stay "
            "under this rule set until it is expired."),
            # Translators: deprecated
            verbose_name=_('Is sticky'))

    comment = models.TextField(blank=True, null=True,
            verbose_name=_('Comment'))

    def __unicode__(self):
        return (
                # Translators: flow access exception in admin (deprecated)
                _("Access exception for '%(user)s' to '%(flow_id)s' "
                "in '%(course)s'") %
                {
                    "user": self.participation.user,
                    "flow_id": self.flow_id,
                    "course": self.participation.course
                    })

    if six.PY3:
        __str__ = __unicode__


class FlowAccessExceptionEntry(models.Model):
    # deprecated

    exception = models.ForeignKey(FlowAccessException,
            related_name="entries",
            verbose_name=_('Exception'), on_delete=models.CASCADE)
    permission = models.CharField(max_length=50,
            choices=FLOW_PERMISSION_CHOICES,
            verbose_name=_('Permission'))

    class Meta:
        # Translators: FlowAccessExceptionEntry (deprecated)
        verbose_name_plural = _("Flow access exception entries")

    def __unicode__(self):
        return self.permission

    if six.PY3:
        __str__ = __unicode__

# }}}


class FlowRuleException(models.Model):
    flow_id = models.CharField(max_length=200, blank=False, null=False,
            verbose_name=_('Flow ID'))
    participation = models.ForeignKey(Participation, db_index=True,
            verbose_name=_('Participation'), on_delete=models.CASCADE)
    expiration = models.DateTimeField(blank=True, null=True,
            verbose_name=_('Expiration'))

    creator = models.ForeignKey(settings.AUTH_USER_MODEL, null=True,
            verbose_name=_('Creator'), on_delete=models.SET_NULL)
    creation_time = models.DateTimeField(default=now, db_index=True,
            verbose_name=_('Creation time'))

    comment = models.TextField(blank=True, null=True,
            verbose_name=_('Comment'))

    kind = models.CharField(max_length=50, blank=False, null=False,
            choices=FLOW_RULE_KIND_CHOICES,
            verbose_name=_('Kind'))
    rule = YAMLField(blank=False, null=False,
            verbose_name=_('Rule'))
    active = models.BooleanField(default=True,
            verbose_name=pgettext_lazy(
                "Is the flow rule exception activated?", "Active"))

    def __unicode__(self):
        return (
                # Translators: For FlowRuleException
                _("%(kind)s exception for '%(user)s' to '%(flow_id)s'"
                "in '%(course)s'")
                % {
                    "kind": self.kind,
                    "user": self.participation.user,
                    "flow_id": self.flow_id,
                    "course": self.participation.course})

    if six.PY3:
        __str__ = __unicode__

    def clean(self):
        super(FlowRuleException, self).clean()

        if (self.kind == flow_rule_kind.grading
                and self.expiration is not None):
            raise ValidationError(_("grading rules may not expire"))

        from course.validation import (
                ValidationError as ContentValidationError,
                validate_session_start_rule,
                validate_session_access_rule,
                validate_session_grading_rule,
                ValidationContext)
        from course.content import (get_course_repo,
                get_course_commit_sha,
                get_flow_desc)

        from relate.utils import dict_to_struct
        rule = dict_to_struct(self.rule)

        repo = get_course_repo(self.participation.course)
        commit_sha = get_course_commit_sha(
                self.participation.course, self.participation)
        ctx = ValidationContext(
                repo=repo,
                commit_sha=commit_sha)

        flow_desc = get_flow_desc(repo,
                self.participation.course,
                self.flow_id, commit_sha)

        tags = None
        grade_identifier = None
        if hasattr(flow_desc, "rules"):
            tags = getattr(flow_desc.rules, "tags", None)
            grade_identifier = flow_desc.rules.grade_identifier

        try:
            if self.kind == flow_rule_kind.start:
                validate_session_start_rule(ctx, six.text_type(self), rule, tags)
            elif self.kind == flow_rule_kind.access:
                validate_session_access_rule(ctx, six.text_type(self), rule, tags)
            elif self.kind == flow_rule_kind.grading:
                validate_session_grading_rule(
                        ctx, six.text_type(self), rule, tags,
                        grade_identifier)
            else:
                # the rule refers to FlowRuleException rule
                raise ValidationError(_("invalid rule kind: ")+self.kind)

        except ContentValidationError as e:
            # the rule refers to FlowRuleException rule
            raise ValidationError(_("invalid existing_session_rules: ")+str(e))

    class Meta:
        verbose_name = _("Flow rule exception")
        verbose_name_plural = _("Flow rule exceptions")

# }}}


# {{{ grading

class GradingOpportunity(models.Model):
    course = models.ForeignKey(Course,
            verbose_name=_('Course'), on_delete=models.CASCADE)

    identifier = models.CharField(max_length=200, blank=False, null=False,
            # Translators: format of identifier for GradingOpportunity
            help_text=_("A symbolic name for this grade. "
            "lower_case_with_underscores, no spaces."),
            verbose_name=_('Grading opportunity ID'),
            validators=[
                RegexValidator(
                    "^"+GRADING_OPP_ID_REGEX+"$",
                    message=_(
                        "Identifier may only contain letters, "
                        "numbers, and hypens ('-').")),
                    ])
    name = models.CharField(max_length=200, blank=False, null=False,
            # Translators: name for GradingOpportunity
            help_text=_("A human-readable identifier for the grade."),
            verbose_name=_('Grading opportunity name'))
    flow_id = models.CharField(max_length=200, blank=True, null=True,
            help_text=_("Flow identifier that this grading opportunity "
            "is linked to, if any"),
            verbose_name=_('Flow ID'))

    aggregation_strategy = models.CharField(max_length=20,
            choices=GRADE_AGGREGATION_STRATEGY_CHOICES,
            # Translators: strategy on how the grading of mutiple sessioins
            # are aggregated.
            verbose_name=_('Aggregation strategy'))

    due_time = models.DateTimeField(default=None, blank=True, null=True,
            verbose_name=_('Due time'))
    creation_time = models.DateTimeField(default=now,
            verbose_name=_('Creation time'))

    shown_in_grade_book = models.BooleanField(default=True,
            verbose_name=_('Shown in grade book'))
    shown_in_participant_grade_book = models.BooleanField(default=True,
            verbose_name=_("Shown in student grade book"))
    result_shown_in_participant_grade_book = models.BooleanField(default=True,
            verbose_name=_("Result shown in student grade book"))

    page_scores_in_participant_gradebook = models.BooleanField(default=False,
            verbose_name=_("Scores for individual pages are shown "
                "in the participants' grade book"))
    hide_superseded_grade_history_before = models.DateTimeField(
            verbose_name=_('Hide superseded grade history before'),
            blank=True, null=True,
            help_text=_(
                'Grade changes dated before this date that are '
                'superseded by later grade changes will not be shown to '
                'participants. '
                'This can help avoid discussions about pre-release grading '
                'adjustments. '
                'May be blank. In that case, the entire grade history is '
                'shown.'))

    class Meta:
        verbose_name = _("Grading opportunity")
        verbose_name_plural = _("Grading opportunities")
        ordering = ("course", "due_time", "identifier")
        unique_together = (("course", "identifier"),)

    def __unicode__(self):
        return (
                # Translators: For GradingOpportunity
                _("%(opportunity_name)s (%(opportunity_id)s) in %(course)s")
                % {
                    "opportunity_name": self.name,
                    "opportunity_id": self.identifier,
                    "course": self.course})

    if six.PY3:
        __str__ = __unicode__

    def get_aggregation_strategy_descr(self):
        return dict(GRADE_AGGREGATION_STRATEGY_CHOICES).get(
                self.aggregation_strategy)


class GradeChange(models.Model):
    """Per 'grading opportunity', each participant may accumulate multiple grades
    that are aggregated according to :attr:`GradingOpportunity.aggregation_strategy`.

    In addition, for each opportunity, grade changes are grouped by their 'attempt'
    identifier, where later grades with the same :attr:`attempt_id` supersede earlier
    ones.
    """
    opportunity = models.ForeignKey(GradingOpportunity,
            verbose_name=_('Grading opportunity'), on_delete=models.CASCADE)

    participation = models.ForeignKey(Participation,
            verbose_name=_('Participation'), on_delete=models.CASCADE)

    state = models.CharField(max_length=50,
            choices=GRADE_STATE_CHANGE_CHOICES,
            # Translators: something like 'status'.
            verbose_name=_('State'))

    attempt_id = models.CharField(max_length=50, null=True, blank=True,
            default="main",
            # Translators: help text of "attempt_id" in GradeChange class
            help_text=_("Grade changes are grouped by their 'attempt ID' "
            "where later grades with the same attempt ID supersede earlier "
            "ones."),
            verbose_name=_('Attempt ID'))

    points = models.DecimalField(max_digits=10, decimal_places=2,
            blank=True, null=True,
            verbose_name=_('Points'))
    max_points = models.DecimalField(max_digits=10, decimal_places=2,
            verbose_name=_('Max points'))

    comment = models.TextField(blank=True, null=True,
            verbose_name=_('Comment'))

    due_time = models.DateTimeField(default=None, blank=True, null=True,
            verbose_name=_('Due time'))

    creator = models.ForeignKey(settings.AUTH_USER_MODEL, null=True,
            verbose_name=_('Creator'), on_delete=models.SET_NULL)
    grade_time = models.DateTimeField(default=now, db_index=True,
            verbose_name=_('Grade time'))

    flow_session = models.ForeignKey(FlowSession, null=True, blank=True,
            related_name="grade_changes",
            verbose_name=_('Flow session'), on_delete=models.SET_NULL)

    class Meta:
        verbose_name = _("Grade change")
        verbose_name_plural = _("Grade changes")
        ordering = ("opportunity", "participation", "grade_time")

    def __unicode__(self):
        # Translators: information for GradeChange
        return _("%(participation)s %(state)s on %(opportunityname)s") % {
            'participation': self.participation,
            'state': self.state,
            'opportunityname': self.opportunity.name}

    if six.PY3:
        __str__ = __unicode__

    def clean(self):
        super(GradeChange, self).clean()

        if self.opportunity.course != self.participation.course:
            raise ValidationError(_("Participation and opportunity must live "
                    "in the same course"))

    def percentage(self):
        # type: () -> Optional[float]

        if (self.max_points is not None
                and self.points is not None
                and self.max_points != 0):
            return 100*self.points/self.max_points
        else:
            return None

    def get_state_desc(self):
        return dict(GRADE_STATE_CHANGE_CHOICES).get(
                self.state)

# }}}


# {{{ grade state machine

class GradeStateMachine(object):
    def __init__(self):
        # type: () -> None
        self.opportunity = None

        self.state = None
        self._clear_grades()
        self.due_time = None
        self.last_graded_time = None
        self.last_report_time = None

        # applies to *all* grade changes
        self._last_grade_change_time = None

    def _clear_grades(self):
        # type: () -> None

        self.state = None
        self.last_grade_time = None
        self.valid_percentages = []  # type: List[GradeChange]
        self.attempt_id_to_gchange = {}  # type: Dict[Text, GradeChange]

    def _consume_grade_change(self, gchange, set_is_superseded):
        # type: (GradeChange, bool) -> None

        if self.opportunity is None:
            opp = self.opportunity = gchange.opportunity
            assert opp is not None
            self.due_time = opp.due_time
        else:
            assert self.opportunity.pk == gchange.opportunity.pk

        assert self.opportunity is not None

        # check that times are increasing
        if self._last_grade_change_time is not None:
            assert gchange.grade_time > self._last_grade_change_time
            self._last_grade_change_time = gchange.grade_time

        if gchange.state == grade_state_change_types.graded:
            if self.state == grade_state_change_types.unavailable:
                raise ValueError(
                        _("cannot accept grade once opportunity has been "
                            "marked 'unavailable'"))
            if self.state == grade_state_change_types.exempt:
                raise ValueError(
                        _("cannot accept grade once opportunity has been "
                        "marked 'exempt'"))

            #if self.due_time is not None and gchange.grade_time > self.due_time:
                #raise ValueError("cannot accept grade after due date")

            self.state = gchange.state
            if gchange.attempt_id is not None:
                if (set_is_superseded and
                        gchange.attempt_id in self.attempt_id_to_gchange):
                    self.attempt_id_to_gchange[gchange.attempt_id] \
                            .is_superseded = True
                self.attempt_id_to_gchange[gchange.attempt_id] \
                        = gchange
            else:
                self.valid_percentages.append(gchange.percentage())

            self.last_graded_time = gchange.grade_time

        elif gchange.state == grade_state_change_types.unavailable:
            self._clear_grades()
            self.state = gchange.state

        elif gchange.state == grade_state_change_types.do_over:
            self._clear_grades()

        elif gchange.state == grade_state_change_types.exempt:
            self._clear_grades()
            self.state = gchange.state

        elif gchange.state == grade_state_change_types.report_sent:
            self.last_report_time = gchange.grade_time

        elif gchange.state == grade_state_change_types.extension:
            self.due_time = gchange.due_time

        elif gchange.state in [
                grade_state_change_types.grading_started,
                grade_state_change_types.retrieved,
                ]:
            pass
        else:
            raise RuntimeError(
                    _("invalid grade change state '%s'") % gchange.state)

    def consume(self, iterable, set_is_superseded=False):
        # type: (Iterable[GradeChange], bool) -> GradeStateMachine

        for gchange in iterable:
            gchange.is_superseded = False
            self._consume_grade_change(gchange, set_is_superseded)

        valid_grade_changes = sorted(
                (gchange
                for gchange in self.attempt_id_to_gchange.values()
                if gchange.percentage() is not None),
                key=lambda gchange: gchange.grade_time)

        self.valid_percentages.extend(
                cast(GradeChange, gchange.percentage())
                for gchange in valid_grade_changes)

        del self.attempt_id_to_gchange

        return self

    def percentage(self):
        # type: () -> Optional[float]

        """
        :return: a percentage of achieved points, or *None*
        """
        if self.opportunity is None or not self.valid_percentages:
            return None

        strategy = self.opportunity.aggregation_strategy

        if strategy == grade_aggregation_strategy.max_grade:
            return max(self.valid_percentages)
        elif strategy == grade_aggregation_strategy.min_grade:
            return min(self.valid_percentages)
        elif strategy == grade_aggregation_strategy.avg_grade:
            return sum(self.valid_percentages)/len(self.valid_percentages)
        elif strategy == grade_aggregation_strategy.use_earliest:
            return self.valid_percentages[0]
        elif strategy == grade_aggregation_strategy.use_latest:
            return self.valid_percentages[-1]
        else:
            raise ValueError(
                    _("invalid grade aggregation strategy '%s'") % strategy)

    def stringify_state(self):
        if self.state is None:
            return u"- ∅ -"
        elif self.state == grade_state_change_types.exempt:
            return "_((exempt))"
        elif self.state == grade_state_change_types.graded:
            if self.valid_percentages:
                result = "%.1f%%" % self.percentage()
                if len(self.valid_percentages) > 1:
                    result += " (/%d)" % len(self.valid_percentages)
                return result
            else:
                return u"- ∅ -"
        else:
            return "_((other state))"

    def stringify_machine_readable_state(self):
        if self.state is None:
            return u"NONE"
        elif self.state == grade_state_change_types.exempt:
            return "EXEMPT"
        elif self.state == grade_state_change_types.graded:
            if self.valid_percentages:
                return "%.3f" % self.percentage()
            else:
                return u"NONE"
        else:
            return u"OTHER_STATE"

    def stringify_percentage(self):
        if self.state == grade_state_change_types.graded:
            if self.valid_percentages:
                return "%.1f" % self.percentage()
            else:
                return u""
        else:
            return ""
# }}}


# {{{ flow <-> grading integration

def get_flow_grading_opportunity(course, flow_id, flow_desc,
        grade_identifier, grade_aggregation_strategy):
    # type: (Course, Text, FlowDesc, Text, Text) -> GradingOpportunity

    default_name = (
            # Translators: display the name of a flow
            _("Flow: %(flow_desc_title)s")
            % {"flow_desc_title": flow_desc.title})

    gopp, created = GradingOpportunity.objects.get_or_create(
            course=course,
            identifier=grade_identifier,
            defaults=dict(
                name=default_name,
                flow_id=flow_id,
                aggregation_strategy=grade_aggregation_strategy,
                ))

    # update gopp.name when flow_desc.title changed
    if not created:
        if gopp.name != default_name:
            gopp.name = default_name
            gopp.save()

    return gopp

# }}}


# {{{ XMPP log

class InstantMessage(models.Model):
    participation = models.ForeignKey(Participation,
            verbose_name=_('Participation'), on_delete=models.CASCADE)
    text = models.CharField(max_length=200,
            verbose_name=_('Text'))
    time = models.DateTimeField(default=now,
            verbose_name=_('Time'))

    class Meta:
        verbose_name = _("Instant message")
        verbose_name_plural = _("Instant messages")
        ordering = ("participation__course", "time")

    def __unicode__(self):
        return "%s: %s" % (self.participation, self.text)

    if six.PY3:
        __str__ = __unicode__

# }}}


# {{{ exams

class Exam(models.Model):
    course = models.ForeignKey(Course,
            verbose_name=_('Course'), on_delete=models.CASCADE)
    description = models.CharField(max_length=200,
            verbose_name=_('Description'))
    flow_id = models.CharField(max_length=200,
            verbose_name=_('Flow ID'))
    active = models.BooleanField(
            default=True,
            verbose_name=_("Active"),
            help_text=_(
                'Currently active, i.e. may be used to log in '
                'via an exam ticket'))
    listed = models.BooleanField(
            verbose_name=_("Listed"),
            default=True,
            help_text=_('Shown in the list of current exams'))

    no_exams_before = models.DateTimeField(
            verbose_name=_('No exams before'))
    no_exams_after = models.DateTimeField(
            null=True, blank=True,
            verbose_name=_('No exams after'))

    class Meta:
        verbose_name = _("Exam")
        verbose_name_plural = _("Exams")
        ordering = ("course", "no_exams_before",)

    def __unicode__(self):
        return _("Exam  %(description)s in %(course)s") % {
                'description': self.description,
                'course': self.course,
                }

    if six.PY3:
        __str__ = __unicode__


class ExamTicket(models.Model):
    exam = models.ForeignKey(Exam,
            verbose_name=_('Exam'), on_delete=models.CASCADE)

    participation = models.ForeignKey(Participation, db_index=True,
            verbose_name=_('Participation'), on_delete=models.CASCADE)

    creator = models.ForeignKey(settings.AUTH_USER_MODEL, null=True,
            verbose_name=_('Creator'), on_delete=models.SET_NULL)
    creation_time = models.DateTimeField(default=now,
            verbose_name=_('Creation time'))
    usage_time = models.DateTimeField(
            verbose_name=_("Usage time"),
            help_text=_('Date and time of first usage of ticket'),
            null=True, blank=True)

    state = models.CharField(max_length=50,
            choices=EXAM_TICKET_STATE_CHOICES,
            verbose_name=_('Exam ticket state'))

    code = models.CharField(max_length=50, db_index=True, unique=True)

    valid_start_time = models.DateTimeField(
            verbose_name=_("End valid period"),
            help_text=_('If not blank, date and time at which this exam ticket '
                'starts being valid/usable'),
            null=True, blank=True)
    valid_end_time = models.DateTimeField(
            verbose_name=_("End valid period"),
            help_text=_('If not blank, date and time at which this exam ticket '
                'stops being valid/usable'),
            null=True, blank=True)
    restrict_to_facility = models.CharField(max_length=200, blank=True, null=True,
            verbose_name=_("Restrict to facility"),
            help_text=_("If not blank, this exam ticket may only be used in the "
                "given facility"))

    class Meta:
        verbose_name = _("Exam ticket")
        verbose_name_plural = _("Exam tickets")
        ordering = ("exam__course", "-creation_time")
        permissions = (
                ("can_issue_exam_tickets", _("Can issue exam tickets to student")),
                )

    def __unicode__(self):
        return _("Exam  ticket for %(participation)s in %(exam)s") % {
                'participation': self.participation,
                'exam': self.exam,
                }

    if six.PY3:
        __str__ = __unicode__

    def clean(self):
        super(ExamTicket, self).clean()

        try:
            if self.exam.course != self.participation.course:
                raise ValidationError(_("Participation and exam must live "
                        "in the same course"))
        except ObjectDoesNotExist:
            pass

# }}}

# vim: foldmethod=marker