Fix out-of-sync sequence for Site ID (#3511)

{{cookiecutter.project_slug}}/{{cookiecutter.project_slug}}/contrib/sites/migrations/0003_set_site_domain_and_name.py

@@ -7,23 +7,52 @@ from django.conf import settings
 from django.db import migrations
 
 
+def _update_or_create_site_with_sequence(site_model, connection, domain, name):
+    """Update or create the site with default ID and keep the DB sequence in sync."""
+    site, created = site_model.objects.update_or_create(
+        id=settings.SITE_ID,
+        defaults={
+            "domain": domain,
+            "name": name,
+        },
+    )
+    if created:
+        # We provided the ID explicitly when creating the Site entry, therefore the DB
+        # sequence to auto-generate them wasn't used and is now out of sync. If we
+        # don't do anything, we'll get a unique constraint violation the next time a
+        # site is created.
+        # To avoid this, we need to manually update DB sequence and make sure it's
+        # greater than the maximum value.
+        max_id = site_model.objects.order_by('-id').first().id
+        with connection.cursor() as cursor:
+            cursor.execute("SELECT last_value from django_site_id_seq")
+            (current_id,) = cursor.fetchone()
+            if current_id <= max_id:
+                cursor.execute(
+                    "alter sequence django_site_id_seq restart with %s",
+                    [max_id + 1],
+                )
+
+
 def update_site_forward(apps, schema_editor):
     """Set site domain and name."""
     Site = apps.get_model("sites", "Site")
-    Site.objects.update_or_create(
-        id=settings.SITE_ID,
-        defaults={
-            "domain": "{{cookiecutter.domain_name}}",
-            "name": "{{cookiecutter.project_name}}",
-        },
+    _update_or_create_site_with_sequence(
+        Site,
+        schema_editor.connection,
+        "{{cookiecutter.domain_name}}",
+        "{{cookiecutter.project_name}}",
     )
 
 
 def update_site_backward(apps, schema_editor):
     """Revert site domain and name to default."""
     Site = apps.get_model("sites", "Site")
-    Site.objects.update_or_create(
-        id=settings.SITE_ID, defaults={"domain": "example.com", "name": "example.com"}
+    _update_or_create_site_with_sequence(
+        Site,
+        schema_editor.connection,
+        "example.com",
+        "example.com",
     )