Early return in migration

arkindex/documents/migrations/0024_migrate_datasource.py

@@ -9,9 +9,12 @@ from arkindex.dataimport.models import RepositoryType
 
 
 def migrate_data_sources(apps, schema_editor):
+    DataSource = apps.get_model('documents', 'DataSource')
+    if not DataSource.objects.exists():
+        return
+
     repo_prefix = os.environ.get('REPOSITORY_PREFIX', 'https://gitlab.com/teklia/workers')
 
-    DataSource = apps.get_model('documents', 'DataSource')
     Element = apps.get_model('documents', 'Element')
     Classification = apps.get_model('documents', 'Classification')
     Transcription = apps.get_model('documents', 'Transcription')