Refactor calculation of recurring events

[timobrembeck]

Motivation

At the moment, the calculation of event recurrences is spread over the

• events API:
  

  integreat-cms/integreat_cms/api/v3/events.py

  Lines 129 to 167 in a15be10

  	def transform_event_recurrences(event_translation, today):
  	"""
  	Yield all future recurrences of the event.
  	:param event_translation: The event translation object which should be converted
  	:type event_translation: ~integreat_cms.cms.models.events.event_translation.EventTranslation
  	:param today: The first date at which event may be yielded
  	:type today: ~datetime.date
  	:return: An iterator over all future recurrences up to ``settings.API_EVENTS_MAX_TIME_SPAN_DAYS``
  	:rtype: Iterator[:class:`~datetime.date`]
  	"""
  	event = event_translation.event
  	recurrence_rule = event.recurrence_rule
  	if not recurrence_rule:
  	return
  	event_is_invalid = (
  	recurrence_rule.recurrence_end_date
  	and recurrence_rule.recurrence_end_date < today
  	)
  	if event_is_invalid:
  	return
  	start_date = event.start_local.date()
  	event_translation.id = None
  	# Calculate all recurrences of this event
  	for recurrence_date in recurrence_rule.iter_after(start_date):
  	if recurrence_date - max(start_date, today) > timedelta(
  	days=settings.API_EVENTS_MAX_TIME_SPAN_DAYS
  	):
  	break
  	if recurrence_date < today or recurrence_date == start_date:
  	continue
  	yield transform_event_translation(event_translation, recurrence_date)

• Event model:
  

  integreat-cms/integreat_cms/cms/models/events/event.py

  Lines 216 to 238 in a15be10

  	def get_occurrences(self, start, end):
  	"""
  	Get occurrences of the event that overlap with ``[start, end]``.
  	Expects ``start < end``.
  	:param start: the begin of the requested interval.
  	:type start: ~datetime.datetime
  	:param end: the end of the requested interval.
  	:type end: ~datetime.datetime
  	:return: start datetimes of occurrences of the event that are in the given timeframe
  	:rtype: list [ ~datetime.datetime ]
  	"""
  	event_start = self.start
  	event_end = self.end
  	if self.is_recurring is not None:
  	return self.recurrence_rule.to_ical_rrule().between(start, end)
  	return (
  	[event_start]
  	if start <= event_start <= end or start <= event_end <= end
  	else []
  	)

• RecurrenceRule model:
  

  integreat-cms/integreat_cms/cms/models/events/recurrence_rule.py

  Lines 126 to 174 in a15be10

  	def advance():
  	"""
  	Get the next occurrence by this rule
  	:return: date objects
  	:rtype: Iterator[:class: `~datetime.date`]
  	"""
  	nonlocal next_recurrence
  	if self.frequency == frequency.DAILY:
  	yield
  	next_recurrence += timedelta(days=1)
  	elif self.frequency == frequency.WEEKLY:
  	# Yield each day of the week that is valid, since
  	# ``interval`` should apply here only on weekly basis
  	for weekday in sorted(self.weekdays_for_weekly):
  	if weekday < next_recurrence.weekday():
  	continue
  	next_recurrence += timedelta(
  	days=weekday - next_recurrence.weekday()
  	)
  	yield
  	# advance to the next monday
  	next_recurrence += timedelta(days=7 - next_recurrence.weekday())
  	elif self.frequency == frequency.MONTHLY:
  	next_recurrence = get_nth_weekday(
  	next_recurrence, self.weekday_for_monthly, self.week_for_monthly
  	)
  	if next_recurrence < start_date:
  	next_recurrence = get_nth_weekday(
  	next_month(next_recurrence),
  	self.weekday_for_monthly,
  	self.week_for_monthly,
  	)
  	yield
  	next_recurrence = next_month(next_recurrence)
  	elif self.frequency == frequency.YEARLY:
  	yield
  	# It is not possible to go simply to the next year if the current date is february 29
  	year_dif = 1
  	while True:
  	try:
  	next_recurrence = next_recurrence.replace(
  	year=next_recurrence.year + year_dif
  	)
  	break
  	except ValueError:
  	year_dif += 1

Proposed Solution

Refactor the code so it

1. Isn't spread over different modules anymore (probably the recurrence rule model is the most sensible place for it)
2. Uses the iCal rrule to calculate the recurrences instead of the custom function

Additional Context

Also, there is a small bug currently that recurring events which spread over multiple days are not shown in the API when the occurrence is ongoing but started e.g. yesterday.