models.py

            # If no student or teacher
            return None


class Event(ExtensibleModel):
    label_ = "event"

    objects = models.Manager.from_queryset(EventQuerySet)()

    title = models.CharField(verbose_name=_("Title"), max_length=255, blank=True, null=True)

    date_start = models.DateField(verbose_name=_("Effective start date of event"), null=True)
    date_end = models.DateField(verbose_name=_("Effective end date of event"), null=True)

    period_from = models.ForeignKey("TimePeriod", on_delete=models.CASCADE, verbose_name=_("Effective start period of event"), related_name="+")
    period_to = models.ForeignKey("TimePeriod", on_delete=models.CASCADE, verbose_name=_("Effective end period of event"), related_name="+")

    groups = models.ManyToManyField("core.Group", related_name="events", verbose_name=_("Groups"))
    rooms = models.ManyToManyField("Room", related_name="events", verbose_name=_("Rooms"))
    teachers = models.ManyToManyField("core.Person", related_name="events", verbose_name=_("Teachers"))

    def __str__(self):
        if self.title:
            return self.title
        else:
            return _("Event {}".format(self.pk))

    class Meta:
        ordering = ["date_start"]
        indexes = [models.Index(fields=["period_from", "period_to", "date_start", "date_end"])]
        verbose_name = _("Event")
        verbose_name_plural = _("Events")


class ExtraLesson(ExtensibleModel):
    week = models.IntegerField(verbose_name=_("Week"), default=CalendarWeek.current_week)
    period = models.ForeignKey("TimePeriod", models.CASCADE, related_name="extra_lessons")

    subject = models.ForeignKey("Subject", on_delete=models.CASCADE, related_name="extra_lessons", verbose_name=_("Subject"))
    groups = models.ManyToManyField("core.Group", related_name="extra_lessons", verbose_name=_("Groups"))
    teachers = models.ManyToManyField("core.Person", related_name="extra_lessons_as_teacher", verbose_name=_("Teachers"))
    room = models.ForeignKey("Room", models.CASCADE, null=True, related_name="extra_lessons", verbose_name=_("Room"))

    comment = models.CharField(verbose_name=_("Comment"), blank=True, null=True, max_length=255)

    def __str__(self):
        return "{}, {}, {}".format(self.week, self.period, self.subject)

    class Meta:
        verbose_name = _("Extra lesson")
        verbose_name_plural = _("Extra lessons")