corporate: Replace unused accessors with database constraint.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

corporate/migrations/0018_customer_cloud_xor_self_hosted.py

@@ -0,0 +1,24 @@
+# Generated by Django 4.2.1 on 2023-05-30 03:16
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        (
+            "corporate",
+            "0017_rename_exempt_from_from_license_number_check_customer_exempt_from_license_number_check",
+        ),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name="customer",
+            constraint=models.CheckConstraint(
+                check=models.Q(
+                    ("realm__isnull", False), ("remote_server__isnull", False), _connector="XOR"
+                ),
+                name="cloud_xor_self_hosted",
+            ),
+        ),
+    ]

corporate/models.py

@@ -3,7 +3,7 @@ from typing import Any, Dict, Optional, Union
 from django.contrib.contenttypes.fields import GenericForeignKey
 from django.contrib.contenttypes.models import ContentType
 from django.db import models
-from django.db.models import CASCADE
+from django.db.models import CASCADE, Q
 
 from zerver.models import Realm, UserProfile
 from zilencer.models import RemoteZulipServer
@@ -28,23 +28,17 @@ class Customer(models.Model):
     # they purchased.
     exempt_from_license_number_check = models.BooleanField(default=False)
 
+    class Meta:
+        constraints = [
+            models.CheckConstraint(
+                check=Q(realm__isnull=False) ^ Q(remote_server__isnull=False),
+                name="cloud_xor_self_hosted",
+            )
+        ]
+
     def __str__(self) -> str:
         return f"{self.realm!r} {self.stripe_customer_id}"
 
-    @property
-    def is_self_hosted(self) -> bool:
-        is_self_hosted = self.remote_server is not None
-        if is_self_hosted:
-            assert self.realm is None
-        return is_self_hosted
-
-    @property
-    def is_cloud(self) -> bool:
-        is_cloud = self.realm is not None
-        if is_cloud:
-            assert self.remote_server is None
-        return is_cloud
-
 
 def get_customer_by_realm(realm: Realm) -> Optional[Customer]:
     return Customer.objects.filter(realm=realm).first()