Remove is_repetition field, add repetition_of fiel to TimeSlot.

program/models.py

@@ -1092,7 +1092,13 @@ class TimeSlot(models.Model):
     start = models.DateTimeField()
     end = models.DateTimeField()
     memo = models.TextField(blank=True)
-    is_repetition = models.BooleanField(default=False)
+    repetition_of = models.ForeignKey(
+        "self",
+        blank=True,
+        null=True,
+        on_delete=models.CASCADE,
+        related_name="repetitions",
+    )
     playlist_id = models.IntegerField(null=True)
     note_id = models.IntegerField(null=True, editable=False)