models: Allow realm_id to be blank.

We cannot provide realm_id for some remote session logs.

corporate/lib/stripe.py

@@ -2340,7 +2340,11 @@ class RemoteRealmBillingSession(BillingSession):  # nocoverage
         *,
         extra_data: Optional[Dict[str, Any]] = None,
     ) -> None:
-        # BUG: This doesn't have a way to pass realm_id !
+        # These audit logs don't use all the fields of `RemoteRealmAuditLog`:
+        #
+        # * remote_id is None because this is not synced from a remote table.
+        # * realm_id is None because we do not aim to store both remote_realm
+        #   and the legacy realm_id field.
         audit_log_event = self.get_audit_log_event(event_type)
         log_data = {
             "server": self.remote_realm.server,

zilencer/migrations/0042_alter_remoterealmauditlog_realm_id.py

@@ -0,0 +1,17 @@
+# Generated by Django 4.2.7 on 2023-11-30 11:24
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zilencer", "0041_remotezulipserver_org_type"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="remoterealmauditlog",
+            name="realm_id",
+            field=models.IntegerField(blank=True, null=True),
+        ),
+    ]

zilencer/models.py

@@ -169,10 +169,13 @@ class RemoteRealmAuditLog(AbstractRealmAuditLog):
 
     server = models.ForeignKey(RemoteZulipServer, on_delete=models.CASCADE)
 
-    # For pre-8.0 servers, we might only have the realm ID.
-    realm_id = models.IntegerField()
-    # With newer servers, we can link to the RemoteRealm object.
+    # With modern Zulip servers, we can link to the RemoteRealm object.
     remote_realm = models.ForeignKey(RemoteRealm, on_delete=models.CASCADE, null=True)
+    # For pre-8.0 servers, we might only have the realm ID and thus no
+    # RemoteRealm object yet. We will eventually be able to drop this
+    # column once all self-hosted servers have upgraded in favor of
+    # just using the foreign key everywhere.
+    realm_id = models.IntegerField(null=True, blank=True)
     # The remote_id field lets us deduplicate data from the remote server
     remote_id = models.IntegerField(null=True)