Fix inconsistency of contest is_organization_private

judge/admin/contest.py

@@ -316,10 +316,6 @@ class ContestAdmin(CompareVersionAdmin):
         # Only rescored if we did not already do so in `save_model`
         if not self._rescored and any(formset.has_changed() for formset in formsets):
             self._rescore(form.cleaned_data["key"])
-        obj = form.instance
-        obj.is_organization_private = obj.organizations.count() > 0
-        obj.is_private = obj.private_contestants.count() > 0
-        obj.save()
 
     def has_change_permission(self, request, obj=None):
         if not request.user.has_perm("judge.edit_own_contest"):

judge/models/contest.py

@@ -2,11 +2,14 @@ from django.core.exceptions import ValidationError
 from django.core.validators import MaxValueValidator, MinValueValidator, RegexValidator
 from django.db import models, transaction
 from django.db.models import CASCADE, Q
+from django.db.models.signals import m2m_changed
 from django.urls import reverse
 from django.utils import timezone
 from django.utils.functional import cached_property
 from django.utils.translation import gettext, gettext_lazy as _
 from django.contrib.contenttypes.fields import GenericRelation
+from django.dispatch import receiver
+
 from jsonfield import JSONField
 from lupa import LuaRuntime
 from moss import (
@@ -658,6 +661,20 @@ class Contest(models.Model, PageVotable, Bookmarkable):
         verbose_name_plural = _("contests")
 
 
+@receiver(m2m_changed, sender=Contest.organizations.through)
+def update_organization_private(sender, instance, **kwargs):
+    if kwargs["action"] in ["post_add", "post_remove", "post_clear"]:
+        instance.is_organization_private = instance.organizations.exists()
+        instance.save(update_fields=["is_organization_private"])
+
+
+@receiver(m2m_changed, sender=Contest.private_contestants.through)
+def update_private(sender, instance, **kwargs):
+    if kwargs["action"] in ["post_add", "post_remove", "post_clear"]:
+        instance.is_private = instance.private_contestants.exists()
+        instance.save(update_fields=["is_private"])
+
+
 class ContestParticipation(models.Model):
     LIVE = 0
     SPECTATE = -1