Add migration to widen username list id to bigint

extlinks/organisations/migrations/0010_widen_username_list_id_to_bigint.py

@@ -0,0 +1,39 @@
+# Generated by Django 4.2.30 on 2026-08-25 15:23
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    """Widen the username_list join table id from int to bigint.
+
+    The table dates from 2019, before DEFAULT_AUTO_FIELD became
+    BigAutoField. Django takes the primary key of an auto-created
+    many-to-many through model from that setting, so a new database
+    builds this column as bigint while production keeps the int column
+    it got in 2019. Migration 0008 widened the concrete models only.
+
+    makemigrations cannot find this difference, because both sides of
+    the comparison come from the same setting. For the same reason the
+    model state is already a BigAutoField, so this needs no
+    state_operations: it only makes the production schema agree.
+
+    The statement is idempotent, and makes no change to a database that
+    is already bigint. The reverse is a no-op, because a real reverse
+    fails or truncates when an id is more than 2**31.
+
+    Bug: T434707
+    """
+
+    dependencies = [
+        ("organisations", "0009_organisation_username_list_updated"),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            # Keep AUTO_INCREMENT here. MODIFY replaces the full column
+            # definition, so the sequence goes away if you remove it.
+            sql="ALTER TABLE organisations_organisation_username_list "
+            "MODIFY id BIGINT NOT NULL AUTO_INCREMENT",
+            reverse_sql=migrations.RunSQL.noop,
+        ),
+    ]