Add classmethod that gets/creates one absence for given time span

aleksis/apps/kolego/models/absence.py

-from datetime import datetime, time
+from datetime import date, datetime, time
 
 from django.db import models
 from django.db.models import Q, QuerySet
@@ -11,7 +11,7 @@ from aleksis.core.managers import (
     RecurrencePolymorphicManager,
 )
 from aleksis.core.mixins import ExtensibleModel
-from aleksis.core.models import FreeBusy
+from aleksis.core.models import FreeBusy, Person
 
 from ..managers import AbsenceQuerySet
 
@@ -148,7 +148,7 @@ class Absence(FreeBusy):
         return f"{self.person} ({self.datetime_start} - {self.datetime_end})"
 
     def save(self, *args, skip_overlap_handling=False, **kwargs):
-        skip_save = False
+        extended_absence = None
         modified_absences = []
 
         if not skip_overlap_handling:
@@ -187,12 +187,12 @@ class Absence(FreeBusy):
 
                 # If overlapping absence has the same reason, just extend it
                 if event_within.reason == self.reason:
-                    skip_save = True
                     event_within.datetime_start = min(
                         new_datetime_start, event_within_datetime_start
                     )
                     event_within.datetime_end = max(new_datetime_end, event_within_datetime_end)
                     event_within.save(skip_overlap_handling=True)
+                    extended_absence = event_within
                     modified_absences.append(event_within)
                 else:
                     if (
@@ -245,10 +245,40 @@ class Absence(FreeBusy):
 
                     modified_absences.append(self)
 
-        if not skip_save:
+        if extended_absence is not None:
+            self._extended_absence = extended_absence
+        else:
             super().save(*args, **kwargs)
         self._modified_absences = modified_absences
 
+    @classmethod
+    def get_for_person_by_datetimes(
+        cls,
+        person: Person,
+        datetime_start: datetime | None = None,
+        datetime_end: datetime | None = None,
+        date_start: date | None = None,
+        date_end: date | None = None,
+        defaults: dict | None = None,
+        **kwargs,
+    ) -> "Absence":
+        data = {"person": person, **kwargs}
+
+        if date_start:
+            data["date_start"] = date_start
+            data["date_end"] = date_end
+        else:
+            data["datetime_start"] = datetime_start
+            data["datetime_end"] = datetime_end
+
+        absence, created = cls.objects.get_or_create(**data, defaults=defaults)
+
+        if created:
+            if hasattr(absence, "_extended_absence"):
+                return absence._extended_absence
+            return absence
+        return absence
+
     class Meta:
         verbose_name = _("Absence")
         verbose_name_plural = _("Absences")