models.py

# -*- coding: utf-8 -*-

from __future__ import division

__copyright__ = "Copyright (C) 2014 Andreas Kloeckner"

__license__ = """
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
"""

from django.db import models
from django.contrib.auth.models import User
from django.utils.timezone import now
from django.core.urlresolvers import reverse
from django.core.exceptions import ValidationError, ObjectDoesNotExist
from django.utils.translation import (
        ugettext_lazy as _ , 
        pgettext_lazy, 
        ugettext_noop,
        string_concat, 
        ugettext,
        )

from course.constants import (  # noqa
        user_status, USER_STATUS_CHOICES,
        participation_role, PARTICIPATION_ROLE_CHOICES,
        participation_status, PARTICIPATION_STATUS_CHOICES,
        flow_permission, FLOW_PERMISSION_CHOICES,
        flow_session_expiration_mode, FLOW_SESSION_EXPIRATION_MODE_CHOICES,
        grade_aggregation_strategy, GRADE_AGGREGATION_STRATEGY_CHOICES,
        grade_state_change_types, GRADE_STATE_CHANGE_CHOICES,
        flow_rule_kind, FLOW_RULE_KIND_CHOICES,
        )


from jsonfield import JSONField
from yamlfield.fields import YAMLField


# {{{ facility

class Facility(models.Model):
    """Data about a facility from where content may be accessed."""

    identifier = models.CharField(max_length=50, unique=True,
            help_text="Format is lower-case-with-hyphens. "
            "Do not use spaces.",
            verbose_name="Facility ID")
    description = models.CharField(max_length=100,
            verbose_name="Facility description")

    class Meta:
        verbose_name = "Facility"
        verbose_name_plural = "Facilities"

    def __unicode__(self):
        return self.identifier


class FacilityIPRange(models.Model):
    """Network data about a facility from where content may be accessed."""

    facility = models.ForeignKey(Facility, related_name="ip_ranges")

    ip_range = models.CharField(
            max_length=200,
            verbose_name="IP range")

    description = models.CharField(max_length=100,
            verbose_name='IP range description')

    class Meta:
        verbose_name = "Facility IP range"

    def clean(self):
        import ipaddr
        try:
            ipaddr.IPNetwork(self.ip_range)
        except Exception as e:
            raise ValidationError({"ip_range": str(e)})

# }}}


# {{{ user status

def get_user_status(user):
    try:
        return user.user_status
    except AttributeError:
        ustatus = UserStatus()
        ustatus.user = user
        ustatus.status = user_status.unconfirmed
        ustatus.save()

        return ustatus


class UserStatus(models.Model):
    user = models.OneToOneField(User, db_index=True, related_name="user_status",
            verbose_name='User ID')
    status = models.CharField(max_length=50,
            choices=USER_STATUS_CHOICES,
            verbose_name='User status')
    sign_in_key = models.CharField(max_length=50,
            help_text="The sign in token sent out in email.",
            null=True, unique=True, db_index=True, blank=True,
            verbose_name='Sign in key')
    key_time = models.DateTimeField(default=now,
            help_text="The time stamp of the sign in token.",
            verbose_name='Key time')

    editor_mode = models.CharField(max_length=20,
            choices=(
                ("default", "Default"),
                ("sublime", "Sublime text"),
                ("emacs", "Emacs"),
                ("vim", "Vim"),
                ),
            default="default",
            # Translators: the text editor used by participants
            verbose_name="Editor mode")

    class Meta:
        verbose_name = "User status"
        verbose_name_plural = "User statuses"
        ordering = ("key_time",)

    def __unicode__(self):
        return "User status for %(user)s" % {'user':self.user}

# }}}


# {{{ course

class Course(models.Model):
    identifier = models.CharField(max_length=200, unique=True,
            help_text="A course identifier. Alphanumeric with dashes, "
            "no spaces. This is visible in URLs and determines the location "
            "on your file system where the course's git repository lives.",
            verbose_name='Course identifier',
            db_index=True)

    hidden = models.BooleanField(
            default=True,
            help_text="Is the course only accessible to course staff?",
            verbose_name='Hidden to student')
    listed = models.BooleanField(
            default=True,
            help_text="Should the course be listed on the main page?",
            verbose_name='Listed on main page')
    accepts_enrollment = models.BooleanField(
            default=True,
            verbose_name='Accepts enrollment')
    valid = models.BooleanField(
            default=True,
            help_text="Whether the course content has passed validation.",
            verbose_name='Valid')

    git_source = models.CharField(max_length=200, blank=True,
            help_text="A Git URL from which to pull course updates. "
            "If you're just starting out, enter "
            "<tt>git://github.com/inducer/relate-sample</tt> "
            "to get some sample content.",
            verbose_name='git source')
    ssh_private_key = models.TextField(blank=True,
            help_text="An SSH private key to use for Git authentication. "
            "Not needed for the sample URL above.",
            verbose_name='SSH private key')

    course_file = models.CharField(max_length=200,
            default="course.yml",
            help_text="Name of a YAML file in the git repository that contains "
            "the root course descriptor.",
            verbose_name='Course file')
    events_file = models.CharField(max_length=200,
            default="events.yml",
            help_text="Name of a YAML file in the git repository that contains "
            "calendar information.",
            verbose_name='Events file')

    enrollment_approval_required = models.BooleanField(
            default=False,
            help_text="If set, each enrolling student must be "
            "individually approved.",
            verbose_name='Enrollment approval required')
    enrollment_required_email_suffix = models.CharField(
            max_length=200, blank=True, null=True,
            help_text="Enrollee's email addresses must end in the "
            "specified suffix, such as '@illinois.edu'.",
            verbose_name='Enrollment required email suffix')

    from_email = models.EmailField(
            help_text="This email address will be used in the 'From' line "
            "of automated emails sent by RELATE."),
            verbose_name='From email')

    notify_email = models.EmailField(
            help_text="This email address will receive "
            "notifications about the course.",
            verbose_name='Notify email')

    # {{{ XMPP

    course_xmpp_id = models.CharField(max_length=200, blank=True, null=True,
            help_text="(Required only if the instant message feature is desired.) "
            "The Jabber/XMPP ID (JID) the course will use to sign in to an "
            "XMPP server.",
            verbose_name='Course xmpp ID')
    course_xmpp_password = models.CharField(max_length=200, blank=True, null=True,
            help_text="(Required only if the instant message feature is desired.) "
            "The password to go with the JID above.",
            verbose_name='Course xmpp password')

    recipient_xmpp_id = models.CharField(max_length=200, blank=True, null=True,
            help_text="(Required only if the instant message feature is desired.) "
            "The JID to which instant messages will be sent.",
            verbose_name='Recipient xmpp ID')

    # }}}

    active_git_commit_sha = models.CharField(max_length=200, null=False,
            blank=False,
            verbose_name='Active git commit SHA')

    participants = models.ManyToManyField(User,
            through='Participation')

    class Meta:
        verbose_name = "Course"
        verbose_name_plural = "Courses"

    def __unicode__(self):
        return self.identifier

    def get_absolute_url(self):
        return reverse("relate-course_page", args=(self.identifier,))

# }}}


# {{{ event

class Event(models.Model):
    """An event is an identifier that can be used to specify dates in
    course content.
    """

    course = models.ForeignKey(Course,
            verbose_name='Course identifier')
    kind = models.CharField(max_length=50,
            help_text="Should be lower_case_with_underscores, no spaces allowed.",
            verbose_name='Kind of event')
    ordinal = models.IntegerField(blank=True, null=True,
            verbose_name='Ordinal of event')

    time = models.DateTimeField(verbose_name='Start time')
    end_time = models.DateTimeField(null=True, blank=True,
            verbose_name='End time')
    all_day = models.BooleanField(default=False,
            # Translators: for when the due time is "All day", how the webpage of a event is displayed.
            help_text="Only affects the rendering in the class calendar, "
            "in that a start time is not shown",
            verbose_name='All day')

    shown_in_calendar = models.BooleanField(default=True,
            verbose_name='Shown in calendar')

    class Meta:
        verbose_name = "Event"
        verbose_name_plural = "Events"
        ordering = ("course", "time")
        unique_together = (("course", "kind", "ordinal"))

    def __unicode__(self):
        if self.ordinal is not None:
            return "%s %s" % (self.kind, self.ordinal)
        else:
            return self.kind

# }}}


# {{{ participation

class ParticipationTag(models.Model):
    course = models.ForeignKey(Course,
            verbose_name='Course identifier')
    name = models.CharField(max_length=100, unique=True,
            help_text="Format is lower-case-with-hyphens. "
            "Do not use spaces.",
            verbose_name='Name of participation tag')

    def clean(self):
        import re
        name_valid_re = re.compile(r"^\w+$")

        if name_valid_re.match(self.name) is None:
            # Translators: "Name" is the name of a ParticipationTag
            raise ValidationError({"name": "Name contains invalid characters."})

    def __unicode__(self):
        return "%s (%s)" % (self.name, self.course)

    class Meta:
        verbose_name = "Participation tag"
        verbose_name_plural = "Participation tags"
        unique_together = (("course", "name"),)
        ordering = ("course", "name")


class Participation(models.Model):
    user = models.ForeignKey(User,
            verbose_name='User ID')
    course = models.ForeignKey(Course, related_name="participations",
            verbose_name='Course identifier')

    enroll_time = models.DateTimeField(default=now,
            verbose_name='Enroll time')
    role = models.CharField(max_length=50,
            choices=PARTICIPATION_ROLE_CHOICES,
            help_text="Instructors may update course content. "
            "Teaching assistants may access and change grade data. "
            "Observers may access analytics. "
            "Each role includes privileges from subsequent roles.",
            verbose_name='Participation role')
    status = models.CharField(max_length=50,
            choices=PARTICIPATION_STATUS_CHOICES,
            verbose_name='Participation status')

    time_factor = models.DecimalField(
            max_digits=10, decimal_places=2,
            default=1,
            help_text="Multiplier for time available on time-limited "
            "flows (time-limited flows are currently unimplemented).",
            verbose_name='Time factor')

    preview_git_commit_sha = models.CharField(max_length=200, null=True,
            blank=True,
            verbose_name='Preview git commit SHA')

    tags = models.ManyToManyField(ParticipationTag, blank=True,
            help_text="NEED HELP TEXT",
            verbose_name='Tags')

    def __unicode__(self):
        # Translators: displayed format of Participation: some user in some course as some role
        return "%(user)s in %(course)s as %(role)s" % {
                'user':self.user, 'course':self.course, 'role':dict(PARTICIPATION_ROLE_CHOICES).get(self.role).lower()}

    class Meta:
        verbose_name = "Participation"
        verbose_name_plural = "Participations"
        unique_together = (("user", "course"),)
        ordering = ("course", "user")


class ParticipationPreapproval(models.Model):
    email = models.EmailField(max_length=254,
            verbose_name='Email')
    course = models.ForeignKey(Course,
            verbose_name='Course identifier')
    role = models.CharField(max_length=50,
            choices=PARTICIPATION_ROLE_CHOICES,
            verbose_name='Role')

    creator = models.ForeignKey(User, null=True,
            verbose_name='Creator')
    creation_time = models.DateTimeField(default=now, db_index=True,
            verbose_name='Creation time')

    def __unicode__(self):
        # Translators: somebody's email in some course in Particiaption Preapproval
        return "%(email)s in %(course)s" % {"email": self.email, "course": self.course}

    class Meta:
        verbose_name = "Participation preapproval"
        verbose_name_plural = "Participation preapprovals"
        unique_together = (("course", "email"),)
        ordering = ("course", "email")

# }}}


class InstantFlowRequest(models.Model):
    course = models.ForeignKey(Course,
            verbose_name='Course identifier')
    flow_id = models.CharField(max_length=200,
            verbose_name='Flow ID')
    start_time = models.DateTimeField(default=now,
            verbose_name='Start time')
    end_time = models.DateTimeField(
            verbose_name='End time')
    cancelled = models.BooleanField(default=False,
            verbose_name='Cancelled')
    
    class Meta:
        verbose_name = "Instant flow request"
        verbose_name_plural = "Instant flow requests"

# {{{ flow session

class FlowSession(models.Model):
    # This looks like it's redundant with 'participation', below--but it's not.
    # 'participation' is nullable.
    course = models.ForeignKey(Course,
            verbose_name='Course identifier')

    participation = models.ForeignKey(Participation, null=True, blank=True,
            db_index=True,
            verbose_name='Participation')
    active_git_commit_sha = models.CharField(max_length=200,
            verbose_name='Active git commit SHA')
    flow_id = models.CharField(max_length=200, db_index=True,
            verbose_name='Flow ID')
    start_time = models.DateTimeField(default=now,
            verbose_name='Start time')
    completion_time = models.DateTimeField(null=True, blank=True,
            verbose_name='Completition time')
    page_count = models.IntegerField(null=True, blank=True,
            verbose_name='Page count')

    in_progress = models.BooleanField(default=None,
            verbose_name='In progress')
    access_rules_tag = models.CharField(max_length=200, null=True,
            blank=True,
            verbose_name='Access rules tag')
    expiration_mode = models.CharField(max_length=20, null=True,
            default=flow_session_expiration_mode.end,
            choices=FLOW_SESSION_EXPIRATION_MODE_CHOICES,
            verbose_name='Expiration mode')

    # Non-normal: These fields can be recomputed, albeit at great expense.
    #
    # Looking up the corresponding GradeChange is also invalid because
    # some flow sessions are not for credit and still have results.

    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,
            blank=True, null=True,
            verbose_name='Max point')
    result_comment = models.TextField(blank=True, null=True,
            verbose_name='Result comment')

    class Meta:
        verbose_name = "Flow session"
        verbose_name_plural = "Flow sessions"
        ordering = ("course", "-start_time")

    def __unicode__(self):
        if self.participation is None:
            return "anonymous session %(session_id)d on '%(flow_id)s'" % {
                    'session_id':self.id,
                    'flow_id':self.flow_id}
        else:
            return "%(user)s's session %(session_id)d on '%(flow_id)s'" % {
                    'user':self.participation.user,
                    'session_id':self.id,
                    'flow_id':self.flow_id}

    def append_comment(self, s):
        if s is None:
            return

        if self.result_comment:
            self.result_comment += "\n" + s
        else:
            self.result_comment = s

    def points_percentage(self):
        if self.points is None:
            return None
        elif self.max_points:
            return 100*self.points/self.max_points
        else:
            return None

    def answer_visits(self):
        # only use this from templates
        from course.flow import assemble_answer_visits
        return assemble_answer_visits(self)

    def last_activity(self):
        for visit in (FlowPageVisit.objects
                .filter(
                    flow_session=self,
                    answer__isnull=False,
                    is_synthetic=False)
                .order_by("-visit_time")
                [:1]):
            return visit.visit_time

        return None

# }}}


# {{{ flow page data

class FlowPageData(models.Model):
    flow_session = models.ForeignKey(FlowSession, related_name="page_data",
            verbose_name='Flow session')
    ordinal = models.IntegerField(null=True, blank=True,
            verbose_name='Ordinal')

    group_id = models.CharField(max_length=200,
            verbose_name='Group ID')
    page_id = models.CharField(max_length=200,
            verbose_name='Page ID')

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

    class Meta:
        verbose_name = "Flow page data"
        verbose_name_plural = "Flow page data"

    def __unicode__(self):
        # flow page data
        return "Data for page '%(group_id)s/%(page_id)s' (ordinal %(ordinal)s) in %(flow_session)s" % {
                'group_id':self.group_id,
                'page_id':self.page_id,
                'ordinal':self.ordinal,
                'flow_session':self.flow_session}

    # Django's templates are a little daft. No arithmetic--really?
    def previous_ordinal(self):
        return self.ordinal - 1

    def next_ordinal(self):
        return self.ordinal + 1

# }}}


# {{{ flow page visit

class FlowPageVisit(models.Model):
    # This is redundant (because the FlowSession is available through
    # page_data), but it helps the admin site understand the link
    # and provide editing.
    flow_session = models.ForeignKey(FlowSession, db_index=True,
            verbose_name='Flow session')

    page_data = models.ForeignKey(FlowPageData, db_index=True,
            verbose_name='Page data')
    visit_time = models.DateTimeField(default=now, db_index=True,
            verbose_name='Visit time')
    remote_address = models.GenericIPAddressField(null=True, blank=True,
            verbose_name='Remote address')

    is_synthetic = models.BooleanField(default=False,
            help_text="Synthetic flow page visits are generated for "
            "unvisited pages once a flow is finished. This is needed "
            "since grade information is attached to a visit, and it "
            "needs a place to go.",
            verbose_name='Is synthetic')

    answer = JSONField(null=True, blank=True,
            # Show correct characters in admin for non ascii languages.
            dump_kwargs={'ensure_ascii': False},            
            # Translators: "Answer" is a Noun.
            verbose_name='Answer')

    # is_submitted_answer may seem redundant with answers being
    # non-NULL, but it isn't. This supports saved (but as
    # yet ungraded) answers.

    # NULL means it's not an answer at all.
    #   (Should coincide with 'answer is None')
    # True means it's a final, submitted answer fit for grading.
    # False means it's just a saved answer.
    is_submitted_answer = models.NullBooleanField(
            # Translators: determine whether the answer is a final, submitted answer fit for grading.
            verbose_name='Is submitted answer')

    def __unicode__(self):
        # Translators: flow page visit
        result = "'%(group_id)s/%(page_id)s' in '%(session)s' on %(time)s" % {
                'group_id':self.page_data.group_id,
                'page_id':self.page_data.page_id,
                'session':self.flow_session,
                'time':self.visit_time}

        if self.answer is not None:
            # Translators: flow page visit: if the answer is provided by user then append the string.
            result += unicode(" (with answer)")

        return result

    class Meta:
        verbose_name = "Flow page visit"
        verbose_name_plural = "Flow page visits"
        # These must be distinguishable, to figure out what came later.
        unique_together = (("page_data", "visit_time"),)

    def get_most_recent_grade(self):
        grades = self.grades.order_by("-grade_time")[:1]

        for grade in grades:
            return grade

        return None

    def get_most_recent_feedback(self):
        grade = self.get_most_recent_grade()

        if grade is None:
            return None
        else:
            return get_feedback_for_grade(grade)

# }}}


#  {{{ flow page visit grade

class FlowPageVisitGrade(models.Model):
    visit = models.ForeignKey(FlowPageVisit, related_name="grades",
            verbose_name='Visit')

    # NULL means 'autograded'
    grader = models.ForeignKey(User, null=True, blank=True,
            verbose_name='Grader')
    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
        return "grade of %(visit)s: %(percentage)s" % {
                'visit':self.visit, 'percentage':self.percentage()}


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')
    grade = models.ForeignKey(FlowPageVisitGrade,
            verbose_name='Grade')

    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):
    FlowPageBulkFeedback.objects.update_or_create(
            page_data=page_data,
            defaults=dict(
                grade=grade,
                bulk_feedback=bulk_feedback_json))


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

    from course.page import AnswerFeedback
    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("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')
    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(User, null=True,
            verbose_name='Creator')
    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):
        # Translators: flow access exception in admin (deprecated)
        return "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}


class FlowAccessExceptionEntry(models.Model):
    # deprecated

    exception = models.ForeignKey(FlowAccessException,
            related_name="entries",
            verbose_name='Exception')
    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

# }}}


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')
    expiration = models.DateTimeField(blank=True, null=True,
            verbose_name='Expiration')

    creator = models.ForeignKey(User, null=True,
            verbose_name='Creator')
    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):
        # Translators: For FlowRuleException
        return "%(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}

    def clean(self):
        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
        if hasattr(flow_desc, "rules"):
            tags = getattr(flow_desc.rules, "tags", None)

        try:
            if self.kind == flow_rule_kind.start:
                validate_session_start_rule(ctx, unicode(self), rule, tags)
            elif self.kind == flow_rule_kind.access:
                validate_session_access_rule(ctx, unicode(self), rule, tags)
            elif self.kind == flow_rule_kind.grading:
                validate_session_grading_rule(ctx, unicode(self), rule, tags)
            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 identifier')

    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')
    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_student_grade_book = models.BooleanField(default=True,
            verbose_name='Shown in student grade book')

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

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


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')

    participation = models.ForeignKey(Participation,
            verbose_name='Participation')

    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,