feat: utilize non-distinct nulls in subscription index

weblate/accounts/migrations/0014_alter_subscription_unique_together_and_more.py

@@ -0,0 +1,31 @@
+# Copyright © Michal Čihař <[email protected]>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+# Generated by Django 5.1.4 on 2024-12-11 11:43
+
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("accounts", "0013_bot_notifications"),
+        ("trans", "0025_alter_announcement_notify"),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name="subscription",
+            unique_together=set(),
+        ),
+        migrations.AddConstraint(
+            model_name="subscription",
+            constraint=models.UniqueConstraint(
+                fields=("notification", "scope", "project", "component", "user"),
+                name="accounts_subscription_notification_unique",
+                nulls_distinct=False,
+            ),
+        ),
+    ]

weblate/accounts/models.py

@@ -187,9 +187,15 @@ class Subscription(models.Model):
     objects = SubscriptionQuerySet.as_manager()
 
     class Meta:
-        unique_together = [("notification", "scope", "project", "component", "user")]
         verbose_name = "Notification subscription"
         verbose_name_plural = "Notification subscriptions"
+        constraints = [
+            models.UniqueConstraint(
+                name="accounts_subscription_notification_unique",
+                fields=("notification", "scope", "project", "component", "user"),
+                nulls_distinct=False,
+            ),
+        ]
 
     def __str__(self) -> str:
         return f"{self.user.username}:{self.get_scope_display()},{self.get_notification_display()} ({self.project},{self.component})"