managers.py

from datetime import date, datetime, timedelta
from enum import Enum
from typing import Dict, Iterable, List, Optional, Union

from django.contrib.sites.managers import CurrentSiteManager as _CurrentSiteManager
from django.db import models
from django.db.models import ExpressionWrapper, F, Func, Q, QuerySet, Value
from django.db.models.fields import DateField
from django.db.models.functions import Concat

from calendarweek import CalendarWeek
from polymorphic.managers import PolymorphicQuerySet

from aleksis.apps.chronos.util.date import week_weekday_from_date, week_weekday_to_date
from aleksis.core.managers import DateRangeQuerySetMixin, SchoolTermRelatedQuerySet
from aleksis.core.models import Group, Person, Room
from aleksis.core.util.core_helpers import get_site_preferences


class ValidityRangeQuerySet(QuerySet, DateRangeQuerySetMixin):
    """Custom query set for validity ranges."""


class ValidityRangeRelatedQuerySet(QuerySet):
    """Custom query set for all models related to validity ranges."""

    def within_dates(self, start: date, end: date) -> "ValidityRangeRelatedQuerySet":
        """Filter for all objects within a date range."""
        return self.filter(validity__date_start__lte=end, validity__date_end__gte=start)

    def in_week(self, wanted_week: CalendarWeek) -> "ValidityRangeRelatedQuerySet":
        """Filter for all objects within a calendar week."""
        return self.within_dates(wanted_week[0], wanted_week[6])

    def on_day(self, day: date) -> "ValidityRangeRelatedQuerySet":
        """Filter for all objects on a certain day."""
        return self.within_dates(day, day)

    def for_validity_range(self, validity_range: "ValidityRange") -> "ValidityRangeRelatedQuerySet":
        return self.filter(validity=validity_range)

    def for_current_or_all(self) -> "ValidityRangeRelatedQuerySet":
        """Get all objects related to current validity range.

        If there is no current validity range, it will return all objects.
        """
        from aleksis.apps.chronos.models import ValidityRange

        current_validity_range = ValidityRange.current
        if current_validity_range:
            return self.for_validity_range(current_validity_range)
        else:
            return self

    def for_current_or_none(self) -> Union["ValidityRangeRelatedQuerySet", None]:
        """Get all objects related to current validity range.

        If there is no current validity range, it will return `None`.
        """
        from aleksis.apps.chronos.models import ValidityRange

        current_validity_range = ValidityRange.current
        if current_validity_range:
            return self.for_validity_range(current_validity_range)
        else:
            return None


class CurrentSiteManager(_CurrentSiteManager):
    use_in_migrations = False


class TimetableType(Enum):
    """Enum for different types of timetables."""

    GROUP = "group"
    TEACHER = "teacher"
    ROOM = "room"

    @classmethod
    def from_string(cls, s: Optional[str]):
        return cls.__members__.get(s.upper())


class LessonPeriodManager(CurrentSiteManager):
    """Manager adding specific methods to lesson periods."""

    def get_queryset(self):
        """Ensure all related lesson data is loaded as well."""
        return (
            super()
            .get_queryset()
            .select_related(
                "lesson",
                "lesson__subject",
                "period",
                "room",
                "lesson__validity",
                "lesson__validity__school_term",
            )
            .prefetch_related(
                "lesson__groups",
                "lesson__groups__parent_groups",
                "lesson__teachers",
                "substitutions",
            )
        )


class LessonSubstitutionManager(CurrentSiteManager):
    """Manager adding specific methods to lesson substitutions."""

    def get_queryset(self):
        """Ensure all related lesson data is loaded as well."""
        return (
            super()
            .get_queryset()
            .select_related(
                "lesson_period",
                "lesson_period__lesson",
                "lesson_period__lesson__subject",
                "subject",
                "lesson_period__period",
                "room",
                "lesson_period__room",
            )
            .prefetch_related(
                "lesson_period__lesson__groups",
                "lesson_period__lesson__groups__parent_groups",
                "teachers",
                "lesson_period__lesson__teachers",
            )
        )


class SupervisionManager(CurrentSiteManager):
    """Manager adding specific methods to supervisions."""

    def get_queryset(self):
        """Ensure all related data is loaded as well."""
        return (
            super()
            .get_queryset()
            .select_related(
                "teacher",
                "area",
                "break_item",
                "break_item__after_period",
                "break_item__before_period",
            )
        )


class SupervisionSubstitutionManager(CurrentSiteManager):
    """Manager adding specific methods to supervision substitutions."""

    def get_queryset(self):
        """Ensure all related data is loaded as well."""
        return (
            super()
            .get_queryset()
            .select_related(
                "teacher",
                "supervision",
                "supervision__teacher",
                "supervision__area",
                "supervision__break_item",
                "supervision__break_item__after_period",
                "supervision__break_item__before_period",
            )
        )


class EventManager(CurrentSiteManager):
    """Manager adding specific methods to events."""

    def get_queryset(self):
        """Ensure all related data is loaded as well."""
        return (
            super()
            .get_queryset()
            .select_related("period_from", "period_to")
            .prefetch_related(
                "groups",
                "groups__school_term",
                "groups__parent_groups",
                "teachers",
                "rooms",
            )
        )


class ExtraLessonManager(CurrentSiteManager):
    """Manager adding specific methods to extra lessons."""

    def get_queryset(self):
        """Ensure all related data is loaded as well."""
        return (
            super()
            .get_queryset()
            .select_related("room", "period", "subject")
            .prefetch_related(
                "groups",
                "groups__school_term",
                "groups__parent_groups",
                "teachers",
            )
        )


class BreakManager(CurrentSiteManager):
    """Manager adding specific methods to breaks."""

    def get_queryset(self):
        """Ensure all related data is loaded as well."""
        return super().get_queryset().select_related("before_period", "after_period")


class WeekQuerySetMixin:
    def annotate_week(self, week: Union[CalendarWeek]):
        """Annotate all lessons in the QuerySet with the number of the provided calendar week."""
        return self.annotate(
            _week=models.Value(week.week, models.IntegerField()),
            _year=models.Value(week.year, models.IntegerField()),
        )

    def alias_week(self, week: Union[CalendarWeek]):
        """Add an alias to all lessons in the QuerySet with the number of the calendar week."""
        return self.alias(
            _week=models.Value(week.week, models.IntegerField()),
            _year=models.Value(week.year, models.IntegerField()),
        )


class GroupByPeriodsMixin:
    def group_by_periods(self, is_week: bool = False) -> dict:
        """Group a QuerySet of objects with attribute period by period numbers and weekdays."""
        per_period = {}
        for obj in self:
            period = obj.period.period
            weekday = obj.period.weekday

            if period not in per_period:
                per_period[period] = [] if not is_week else {}

            if is_week and weekday not in per_period[period]:
                per_period[period][weekday] = []

            if not is_week:
                per_period[period].append(obj)
            else:
                per_period[period][weekday].append(obj)

        return per_period


class LessonDataQuerySet(models.QuerySet, WeekQuerySetMixin):
    """Overrides default QuerySet to add specific methods for lesson data."""

    # Overridden in the subclasses. Swaps the paths to the base lesson period
    # and to any substitutions depending on whether the query is run on a
    # lesson period or a substitution
    _period_path = None
    _subst_path = None

    def within_dates(self, start: date, end: date):
        """Filter for all lessons within a date range."""
        return self.filter(
            **{
                self._period_path + "lesson__validity__date_start__lte": start,
                self._period_path + "lesson__validity__date_end__gte": end,
            }
        )

    def in_week(self, wanted_week: CalendarWeek):
        """Filter for all lessons within a calendar week."""
        return self.within_dates(
            wanted_week[0] + timedelta(days=1) * (F(self._period_path + "period__weekday")),
            wanted_week[0] + timedelta(days=1) * (F(self._period_path + "period__weekday")),
        ).annotate_week(wanted_week)

    def on_day(self, day: date):
        """Filter for all lessons on a certain day."""
        week, weekday = week_weekday_from_date(day)

        return (
            self.within_dates(day, day)
            .filter(**{self._period_path + "period__weekday": weekday})
            .annotate_week(week)
        )

    def at_time(self, when: Optional[datetime] = None):
        """Filter for the lessons taking place at a certain point in time."""
        now = when or datetime.now()
        week, weekday = week_weekday_from_date(now.date())

        return self.filter(
            **{
                self._period_path + "lesson__validity__date_start__lte": now.date(),
                self._period_path + "lesson__validity__date_end__gte": now.date(),
                self._period_path + "period__weekday": now.weekday(),
                self._period_path + "period__time_start__lte": now.time(),
                self._period_path + "period__time_end__gte": now.time(),
            }
        ).annotate_week(week)

    def filter_participant(self, person: Union[Person, int]):
        """Filter for all lessons a participant (student) attends."""
        return self.filter(Q(**{self._period_path + "lesson__groups__members": person}))

    def filter_group(self, group: Union[Group, int]):
        """Filter for all lessons a group (class) regularly attends."""
        if isinstance(group, int):
            group = Group.objects.get(pk=group)

        if group.parent_groups.all():
            # Prevent to show lessons multiple times
            return self.filter(Q(**{self._period_path + "lesson__groups": group}))
        else:
            return self.filter(
                Q(**{self._period_path + "lesson__groups": group})
                | Q(**{self._period_path + "lesson__groups__parent_groups": group})
            )

    def filter_groups(self, groups: Iterable[Group]) -> QuerySet:
        """Filter for all lessons one of the groups regularly attends."""
        return self.filter(
            Q(**{self._period_path + "lesson__groups__in": groups})
            | Q(**{self._period_path + "lesson__groups__parent_groups__in": groups})
        )

    def filter_teacher(self, teacher: Union[Person, int], is_smart: bool = True):
        """Filter for all lessons given by a certain teacher."""
        qs1 = self.filter(**{self._period_path + "lesson__teachers": teacher})
        qs2 = self.filter(
            **{
                self._subst_path + "teachers": teacher,
                self._subst_path + "week": F("_week"),
                self._subst_path + "year": F("_year"),
            }
        )

        if is_smart:
            return qs1.union(qs2)
        else:
            return qs1

    def filter_room(self, room: Union["Room", int], is_smart: bool = True):
        """Filter for all lessons taking part in a certain room."""
        qs1 = self.filter(**{self._period_path + "room": room})
        qs2 = self.filter(
            **{
                self._subst_path + "room": room,
                self._subst_path + "week": F("_week"),
                self._subst_path + "year": F("_year"),
            }
        )

        if is_smart:
            return qs1.union(qs2)
        else:
            return qs1

    def filter_from_type(
        self, type_: TimetableType, obj: Union[Person, Group, "Room", int], is_smart: bool = True
    ) -> Optional[models.QuerySet]:
        """Filter lesson data for a group, teacher or room by provided type."""
        if type_ == TimetableType.GROUP:
            return self.filter_group(obj)
        elif type_ == TimetableType.TEACHER:
            return self.filter_teacher(obj, is_smart=is_smart)
        elif type_ == TimetableType.ROOM:
            return self.filter_room(obj, is_smart=is_smart)
        else:
            return None

    def filter_from_person(self, person: Person) -> Optional[models.QuerySet]:
        """Filter lesson data for a person."""
        type_ = person.timetable_type

        if type_ == TimetableType.TEACHER:
            # Teacher

            return self.filter_teacher(person)

        elif type_ == TimetableType.GROUP:
            # Student

            return self.filter_participant(person)

        else:
            # If no student or teacher
            return None

    def daily_lessons_for_person(
        self, person: Person, wanted_day: date
    ) -> Optional[models.QuerySet]:
        """Filter lesson data on a day by a person."""
        if person.timetable_type is None:
            return None

        lesson_periods = self.on_day(wanted_day).filter_from_person(person)

        return lesson_periods

    def group_by_validity(self) -> Dict["ValidityRange", List["LessonPeriod"]]:
        """Group lesson periods by validity range as dictionary."""
        lesson_periods_by_validity = {}
        for lesson_period in self:
            lesson_periods_by_validity.setdefault(lesson_period.lesson.validity, [])
            lesson_periods_by_validity[lesson_period.lesson.validity].append(lesson_period)
        return lesson_periods_by_validity

    def next_lesson(
        self, reference: "LessonPeriod", offset: Optional[int] = 1
    ) -> Optional["LessonPeriod"]:
        """Get another lesson in an ordered set of lessons.

        By default, it returns the next lesson in the set. By passing the offset argument,
        the n-th next lesson can be selected. By passing a negative number, the n-th
        previous lesson can be selected.

        This function will handle week, year and validity range changes automatically
        if the queryset contains enough lesson data.
        """
        # Group lesson periods by validity to handle validity range changes correctly
        lesson_periods_by_validity = self.group_by_validity()
        validity_ranges = list(lesson_periods_by_validity.keys())

        # List with lesson periods in the validity range of the reference lesson period
        current_lesson_periods = lesson_periods_by_validity[reference.lesson.validity]
        pks = [lesson_period.pk for lesson_period in current_lesson_periods]

        # Position of the reference lesson period
        index = pks.index(reference.id)

        next_index = index + offset
        if next_index > len(pks) - 1:
            next_index %= len(pks)
            week = reference._week + 1
        elif next_index < 0:
            next_index = len(pks) + next_index
            week = reference._week - 1
        else:
            week = reference._week

        # Check if selected week makes a year change necessary
        year = reference._year
        if week < 1:
            year -= 1
            week = CalendarWeek.get_last_week_of_year(year).week
        elif week > CalendarWeek.get_last_week_of_year(year).week:
            year += 1
            week = 1

        # Get the next lesson period in this validity range and it's date
        # to check whether the validity range has to be changed
        week = CalendarWeek(week=week, year=year)
        next_lesson_period = current_lesson_periods[next_index]
        next_lesson_period_date = week_weekday_to_date(week, next_lesson_period.period.weekday)

        validity_index = validity_ranges.index(next_lesson_period.lesson.validity)

        # If date of next lesson period is out of validity range (smaller) ...
        if next_lesson_period_date < next_lesson_period.lesson.validity.date_start:
            # ... we have to get the lesson period from the previous validity range
            if validity_index == 0:
                # There are no validity ranges (and thus no lessons)
                # in the school term before this lesson period
                return None

            # Get new validity range and last lesson period of this validity range
            new_validity = validity_ranges[validity_index - 1]
            next_lesson_period = lesson_periods_by_validity[new_validity][-1]

            # Build new week with the date from the new validity range/lesson period
            week = CalendarWeek(
                week=new_validity.date_end.isocalendar()[1], year=new_validity.date_end.year
            )

        # If date of next lesson period is out of validity range (larger) ...
        elif next_lesson_period_date > next_lesson_period.lesson.validity.date_end:
            # ... we have to get the lesson period from the next validity range
            if validity_index >= len(validity_ranges) - 1:
                # There are no validity ranges (and thus no lessons)
                # in the school term after this lesson period
                return None

            # Get new validity range and first lesson period of this validity range
            new_validity = validity_ranges[validity_index + 1]
            next_lesson_period = lesson_periods_by_validity[new_validity][0]

            # Build new week with the date from the new validity range/lesson period
            week = CalendarWeek(
                week=new_validity.date_start.isocalendar()[1], year=new_validity.date_start.year
            )

        # Do a new query here to be able to annotate the new week
        return self.annotate_week(week).get(pk=next_lesson_period.pk)


class LessonPeriodQuerySet(LessonDataQuerySet, GroupByPeriodsMixin):
    """QuerySet with custom query methods for lesson periods."""

    _period_path = ""
    _subst_path = "substitutions__"


class LessonSubstitutionQuerySet(LessonDataQuerySet):
    """QuerySet with custom query methods for substitutions."""

    _period_path = "lesson_period__"
    _subst_path = ""

    def within_dates(self, start: date, end: date):
        """Filter for all substitutions within a date range."""
        start_week = CalendarWeek.from_date(start)
        end_week = CalendarWeek.from_date(end)
        return self.filter(
            week__gte=start_week.week,
            week__lte=end_week.week,
            year__gte=start_week.year,
            year__lte=end_week.year,
        ).filter(
            Q(
                week=start_week.week,
                year=start_week.year,
                lesson_period__period__weekday__gte=start.weekday(),
            )
            | Q(
                week=end_week.week,
                year=end_week.year,
                lesson_period__period__weekday__lte=end.weekday(),
            )
            | (
                ~Q(week=start_week.week, year=start_week.year)
                & ~Q(week=end_week.week, year=end_week.year)
            )
        )

    def in_week(self, wanted_week: CalendarWeek):
        """Filter for all lessons within a calendar week."""
        return self.filter(week=wanted_week.week, year=wanted_week.year).annotate_week(wanted_week)

    def on_day(self, day: date):
        """Filter for all lessons on a certain day."""
        week, weekday = week_weekday_from_date(day)

        return self.in_week(week).filter(lesson_period__period__weekday=weekday)

    def at_time(self, when: Optional[datetime] = None):
        """Filter for the lessons taking place at a certain point in time."""
        now = when or datetime.now()

        return self.on_day(now.date()).filter(
            lesson_period__period__time_start__lte=now.time(),
            lesson_period__period__time_end__gte=now.time(),
        )

    def affected_lessons(self):
        """Return all lessons which are affected by selected substitutions."""
        from .models import Lesson  # noaq

        return Lesson.objects.filter(lesson_periods__substitutions__in=self).distinct()

    def affected_teachers(self):
        """Get affected teachers.

        Return all teachers which are affected by
        selected substitutions (as substituted or substituting).
        """
        return (
            Person.objects.filter(
                Q(lessons_as_teacher__in=self.affected_lessons()) | Q(lesson_substitutions__in=self)
            )
            .distinct()
            .order_by("short_name")
        )

    def affected_groups(self):
        """Return all groups which are affected by selected substitutions."""
        return (
            Group.objects.filter(lessons__in=self.affected_lessons())
            .distinct()
            .order_by("short_name")
        )


class DateRangeQuerySetMixin:
    """QuerySet with custom query methods for models with date and period ranges.

    Filterable fields: date_start, date_end, period_from, period_to
    """

    def within_dates(self, start: date, end: date):
        """Filter for all events within a date range."""
        return self.filter(date_start__lte=end, date_end__gte=start)

    def in_week(self, wanted_week: CalendarWeek):
        """Filter for all events within a calendar week."""
        return self.within_dates(wanted_week[0], wanted_week[6])

    def on_day(self, day: date):
        """Filter for all events on a certain day."""
        return self.within_dates(day, day)

    def at_time(self, when: Optional[datetime] = None):
        """Filter for the events taking place at a certain point in time."""
        now = when or datetime.now()

        return self.on_day(now.date()).filter(
            period_from__time_start__lte=now.time(), period_to__time_end__gte=now.time()
        )

    def exclude_holidays(self, holidays: Iterable["Holiday"]) -> QuerySet:
        """Exclude all objects which are in the provided holidays."""
        q = Q()
        for holiday in holidays:
            q = q | Q(date_start__lte=holiday.date_end, date_end__gte=holiday.date_start)
        return self.exclude(q)


class AbsenceQuerySet(DateRangeQuerySetMixin, SchoolTermRelatedQuerySet):
    """QuerySet with custom query methods for absences."""

    def absent_teachers(self):
        return Person.objects.filter(absences__in=self).distinct().order_by("short_name")

    def absent_groups(self):
        return Group.objects.filter(absences__in=self).distinct().order_by("short_name")

    def absent_rooms(self):
        return Person.objects.filter(absences__in=self).distinct().order_by("short_name")


class HolidayQuerySet(QuerySet, DateRangeQuerySetMixin):
    """QuerySet with custom query methods for holidays."""

    def get_all_days(self) -> List[date]:
        """Get all days included in the selected holidays."""
        holiday_days = []
        for holiday in self:
            holiday_days += list(holiday.get_days())
        return holiday_days


class SupervisionQuerySet(ValidityRangeRelatedQuerySet, WeekQuerySetMixin):
    """QuerySet with custom query methods for supervisions."""

    def filter_by_weekday(self, weekday: int):
        """Filter supervisions by weekday."""
        return self.filter(
            Q(break_item__before_period__weekday=weekday)
            | Q(break_item__after_period__weekday=weekday)
        )

    def filter_by_teacher(self, teacher: Union[Person, int]):
        """Filter for all supervisions given by a certain teacher."""
        if self.count() > 0:
            if hasattr(self[0], "_week"):
                week = CalendarWeek(week=self[0]._week, year=self[0]._year)
            else:
                week = CalendarWeek.current_week()

            dates = [week[w] for w in range(0, 7)]

            return self.filter(
                Q(substitutions__teacher=teacher, substitutions__date__in=dates)
                | Q(teacher=teacher)
            )

        return self


class TimetableQuerySet(models.QuerySet):
    """Common query set methods for objects in timetables.

    Models need following fields:
    - groups
    - teachers
    - rooms (_multiple_rooms=True)/room (_multiple_rooms=False)
    """

    _multiple_rooms = True

    def filter_participant(self, person: Union[Person, int]):
        """Filter for all objects a participant (student) attends."""
        return self.filter(Q(groups__members=person))

    def filter_group(self, group: Union[Group, int]):
        """Filter for all objects a group (class) attends."""
        if isinstance(group, int):
            group = Group.objects.get(pk=group)

        if group.parent_groups.all():
            # Prevent to show lessons multiple times
            return self.filter(groups=group)
        else:
            return self.filter(Q(groups=group) | Q(groups__parent_groups=group))

    def filter_groups(self, groups: Iterable[Group]) -> QuerySet:
        """Filter for all objects one of the groups attends."""
        return self.filter(Q(groups__in=groups) | Q(groups__parent_groups__in=groups)).distinct()

    def filter_teacher(self, teacher: Union[Person, int]):
        """Filter for all lessons given by a certain teacher."""
        return self.filter(teachers=teacher)

    def filter_room(self, room: Union["Room", int]):
        """Filter for all objects taking part in a certain room."""
        if self._multiple_rooms:
            return self.filter(rooms=room)
        else:
            return self.filter(room=room)

    def filter_from_type(
        self, type_: TimetableType, obj: Union[Group, Person, "Room", int]
    ) -> Optional[models.QuerySet]:
        """Filter data for a group, teacher or room by provided type."""
        if type_ == TimetableType.GROUP:
            return self.filter_group(obj)
        elif type_ == TimetableType.TEACHER:
            return self.filter_teacher(obj)
        elif type_ == TimetableType.ROOM:
            return self.filter_room(obj)
        else:
            return None

    def filter_from_person(self, person: Person) -> Optional[models.QuerySet]:
        """Filter data by person."""
        type_ = person.timetable_type

        if type_ == TimetableType.TEACHER:
            # Teacher

            return self.filter_teacher(person)

        elif type_ == TimetableType.GROUP:
            # Student

            return self.filter_participant(person)

        else:
            # If no student or teacher
            return None


class EventQuerySet(DateRangeQuerySetMixin, SchoolTermRelatedQuerySet, TimetableQuerySet):
    """QuerySet with custom query methods for events."""

    def annotate_day(self, day: date):
        """Annotate all events in the QuerySet with the provided date."""
        return self.annotate(_date=models.Value(day, models.DateField()))

    def alias_day(self, day: date):
        """Add an alias to all events in the QuerySet with the provided date."""
        return self.alias(_date=models.Value(day, models.DateField()))


class ExtraLessonQuerySet(TimetableQuerySet, SchoolTermRelatedQuerySet, GroupByPeriodsMixin):
    """QuerySet with custom query methods for extra lessons."""

    _multiple_rooms = False

    def within_dates(self, start: date, end: date):
        """Filter all extra lessons within a specific time range."""
        return self.alias_day().filter(day__gte=start, day__lte=end)

    def on_day(self, day: date):
        """Filter all extra lessons on a day."""
        return self.within_dates(day, day)

    def _get_weekday_to_date(self):
        """Get DB function to convert a weekday to a date."""
        return ExpressionWrapper(
            Func(
                Concat(F("year"), F("week")),
                Value("IYYYIW"),
                output_field=DateField(),
                function="TO_DATE",
            )
            + F("period__weekday"),
            output_field=DateField(),
        )

    def annotate_day(self):
        return self.annotate(day=self._get_weekday_to_date())

    def alias_day(self):
        return self.alias(day=self._get_weekday_to_date())

    def exclude_holidays(self, holidays: Iterable["Holiday"]) -> QuerySet:
        """Exclude all extra lessons which are in the provided holidays."""
        q = Q()
        for holiday in holidays:
            q = q | Q(day__lte=holiday.date_end, day__gte=holiday.date_start)
        return self.alias_day().exclude(q)


class GroupPropertiesMixin:
    """Mixin for common group properties.

    Necessary method: `get_groups`
    """

    @property
    def group_names(self, sep: Optional[str] = ", ") -> str:
        return sep.join([group.short_name for group in self.get_groups()])

    @property
    def group_short_names(self, sep: Optional[str] = ", ") -> str:
        return sep.join([group.short_name for group in self.get_groups()])

    @property
    def groups_to_show(self) -> models.QuerySet:
        groups = self.get_groups()
        if (
            groups.count() == 1
            and groups[0].parent_groups.all()
            and get_site_preferences()["chronos__use_parent_groups"]
        ):
            return groups[0].parent_groups.all()
        else:
            return groups

    @property
    def groups_to_show_names(self, sep: Optional[str] = ", ") -> str:
        return sep.join([group.short_name for group in self.groups_to_show])

    @property
    def groups_to_show_short_names(self, sep: Optional[str] = ", ") -> str:
        return sep.join([group.short_name for group in self.groups_to_show])


class TeacherPropertiesMixin:
    """Mixin for common teacher properties.

    Necessary method: `get_teachers`
    """

    @property
    def teacher_names(self, sep: Optional[str] = ", ") -> str:
        return sep.join([teacher.full_name for teacher in self.get_teachers()])

    @property
    def teacher_short_names(self, sep: str = ", ") -> str:
        return sep.join([teacher.short_name for teacher in self.get_teachers()])


class RoomPropertiesMixin:
    """Mixin for common room properties.

    Necessary method: `get_rooms`
    """

    @property
    def room_names(self, sep: Optional[str] = ", ") -> str:
        return sep.join([room.name for room in self.get_rooms()])

    @property
    def room_short_names(self, sep: str = ", ") -> str:
        return sep.join([room.short_name for room in self.get_rooms()])


class LessonEventQuerySet(PolymorphicQuerySet):
    """Queryset with special query methods for lesson events."""

    def for_teacher(self, teacher: Union[int, Person]):
        amended = self.filter(Q(amended_by__isnull=False) & (Q(teachers=teacher))).values_list(
            "amended_by__pk", flat=True
        )
        return self.filter(Q(teachers=teacher) | Q(pk__in=amended)).distinct()

    def for_group(self, group: Union[int, Group]):
        amended = self.filter(
            Q(amended_by__isnull=False) & (Q(groups=group) | Q(groups__parent_groups=group))
        ).values_list("amended_by__pk", flat=True)
        return self.filter(
            Q(groups=group) | Q(groups__parent_groups=group) | Q(pk__in=amended)
        ).distinct()

    def for_room(self, room: Union[int, Room]):
        amended = self.filter(Q(amended_by__isnull=False) & (Q(rooms=room))).values_list(
            "amended_by__pk", flat=True
        )
        return self.filter(Q(rooms=room) | Q(pk__in=amended)).distinct()

    def for_person(self, person: Union[int, Person]):
        amended = self.filter(
            Q(amended_by__isnull=False) & (Q(teachers=person) | Q(groups__members=person))
        ).values_list("amended_by__pk", flat=True)
        return self.filter(
            Q(teachers=person) | Q(groups__members=person) | Q(pk__in=amended)
        ).distinct()