models: Fix incorrect on_delete=CASCADE values for deleted streams.

In theory, we never delete streams, but if we did delete the
notifications stream for a realm, we certainly shouldn't delete the
realm.

zerver/migrations/0324_fix_deletion_cascade_behavior.py

@@ -0,0 +1,56 @@
+# Generated by Django 3.2 on 2021-05-07 01:00
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zerver", "0323_show_starred_message_counts"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="realm",
+            name="notifications_stream",
+            field=models.ForeignKey(
+                blank=True,
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                related_name="+",
+                to="zerver.stream",
+            ),
+        ),
+        migrations.AlterField(
+            model_name="realm",
+            name="signup_notifications_stream",
+            field=models.ForeignKey(
+                blank=True,
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                related_name="+",
+                to="zerver.stream",
+            ),
+        ),
+        migrations.AlterField(
+            model_name="userprofile",
+            name="default_events_register_stream",
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                related_name="+",
+                to="zerver.stream",
+            ),
+        ),
+        migrations.AlterField(
+            model_name="userprofile",
+            name="default_sending_stream",
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                related_name="+",
+                to="zerver.stream",
+            ),
+        ),
+    ]

zerver/models.py

@@ -375,14 +375,14 @@ class Realm(models.Model):
         related_name="+",
         null=True,
         blank=True,
-        on_delete=CASCADE,
+        on_delete=models.SET_NULL,
     )
     signup_notifications_stream: Optional["Stream"] = models.ForeignKey(
         "Stream",
         related_name="+",
         null=True,
         blank=True,
-        on_delete=CASCADE,
+        on_delete=models.SET_NULL,
     )
 
     MESSAGE_RETENTION_SPECIAL_VALUES_MAP = {
@@ -1266,13 +1266,13 @@ class UserProfile(AbstractBaseUser, PermissionsMixin):
         "zerver.Stream",
         null=True,
         related_name="+",
-        on_delete=CASCADE,
+        on_delete=models.SET_NULL,
     )
     default_events_register_stream: Optional["Stream"] = models.ForeignKey(
         "zerver.Stream",
         null=True,
         related_name="+",
-        on_delete=CASCADE,
+        on_delete=models.SET_NULL,
     )
     default_all_public_streams: bool = models.BooleanField(default=False)