Add migration.

biscuit/apps/chronos/migrations/0004_room_name_not_unique.py

+# Generated by Django 2.2.5 on 2019-09-14 16:17
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('chronos', '0003_substitution_cancelled_or_subject'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='room',
+            unique_together={('school', 'short_name')},
+        ),
+    ]