misc: create timeslot virtual id on-demand (#42147)

chrono/agendas/models.py

@@ -31,6 +31,7 @@ from django.urls import reverse
 from django.utils.dates import WEEKDAYS
 from django.utils.encoding import force_text
 from django.utils.formats import date_format
+from django.utils.functional import cached_property
 from django.utils.text import slugify
 from django.utils.timezone import localtime, now, make_aware, make_naive, is_aware
 from django.utils.translation import ugettext_lazy as _
@@ -340,9 +341,15 @@ class TimeSlot(object):
         self.start_datetime = start_datetime
         self.end_datetime = start_datetime + datetime.timedelta(minutes=meeting_type.duration)
         self.meeting_type = meeting_type
-        self.id = '%s:%s' % (meeting_type.id or meeting_type.slug, start_datetime.strftime('%Y-%m-%d-%H%M'))
         self.desk = desk
 
+    @cached_property
+    def id(self):
+        return '%s:%s' % (
+            self.meeting_type.id or self.meeting_type.slug,
+            self.start_datetime.strftime('%Y-%m-%d-%H%M'),
+        )
+
     def __str__(self):
         return date_format(self.start_datetime, format='DATETIME_FORMAT')