Update migration

- Use les `bd_alias` de Django
- N'ajoute le 'before <date> """<ancien_token>"""' dans la liste des tokens
  si le champ token était précédement vide.

bda/migrations/0004_add_tirage_switch.py

@@ -4,14 +4,17 @@ from __future__ import unicode_literals
 from django.db import migrations, models
 from django.utils import timezone
 
+
 def forwards_func(apps, schema_editor):
     Tirage = apps.get_model("bda", "Tirage")
     db_alias = schema_editor.connection.alias
-    for tirage in Tirage.objects.all():
-        tirage.tokens = "Before %s\n\"\"\"%s\"\"\"\n" % (
-                timezone.now().strftime("%y-%m-%d %H:%M:%S"), 
-                tirage.tokens)
-        tirage.save()
+    for tirage in Tirage.objects.using(db_alias).all():
+        if tirage.tokens:
+            tirage.tokens = "Before %s\n\"\"\"%s\"\"\"\n" % (
+                    timezone.now().strftime("%y-%m-%d %H:%M:%S"),
+                    tirage.tokens)
+            tirage.save()
+
 
 class Migration(migrations.Migration):