Resolve "Carry over the data of the first lesson for double (or triple, ...) lessons"

Closes #64 (closed)

aleksis/apps/alsijil/models.py

 from django.db import models
 from django.utils.translation import gettext_lazy as _
 
+from calendarweek import CalendarWeek
+
+from aleksis.apps.chronos.models import LessonPeriod
 from aleksis.core.mixins import ExtensibleModel
+from aleksis.core.util.core_helpers import get_site_preferences
 
 
 def isidentifier(value: str) -> bool:
@@ -102,6 +106,47 @@ class LessonDocumentation(ExtensibleModel):
         verbose_name=_("Group note"), max_length=200, blank=True
     )
 
+    def _carry_over_data(self):
+        """Carry over data to the next lesson, if exists and data are not already set.
+
+        Can be deactivated using site preference ``alsijil__carry_over``.
+        """
+        following_periods = LessonPeriod.objects.filter(
+            lesson=self.lesson_period.lesson,
+            period__weekday=self.lesson_period.period.weekday,
+            period__period__gt=self.lesson_period.period.period,
+        )
+        for period in following_periods:
+            lesson_documentation = period.get_or_create_lesson_documentation(
+                CalendarWeek(
+                    week=self.week, year=self.lesson_period.lesson.get_year(self.week),
+                )
+            )
+
+            changed = False
+
+            if not lesson_documentation.topic:
+                lesson_documentation.topic = self.topic
+                changed = True
+
+            if not lesson_documentation.homework:
+                lesson_documentation.homework = self.homework
+                changed = True
+
+            if not lesson_documentation.group_note:
+                lesson_documentation.group_note = self.group_note
+                changed = True
+
+            if changed:
+                lesson_documentation.save()
+
+    def save(self, *args, **kwargs):
+        if get_site_preferences()["alsijil__carry_over"] and (
+            self.topic or self.homework or self.group_note
+        ):
+            self._carry_over_data()
+        super().save(*args, **kwargs)
+
     class Meta:
         verbose_name = _("Lesson documentation")
         verbose_name_plural = _("Lesson documentations")