Use a signal to prevent a user that the note balance is negative

apps/note/signals.py

@@ -1,6 +1,8 @@
 # Copyright (C) 2018-2020 by BDE ENS Paris-Saclay
 # SPDX-License-Identifier: GPL-3.0-or-later
 
+from django.utils import timezone
+
 
 def save_user_note(instance, raw, **_kwargs):
     """
@@ -25,6 +27,16 @@ def save_club_note(instance, raw, **_kwargs):
         instance.note.save()
 
 
+def pre_save_note(instance, raw, **_kwargs):
+    if not raw and instance.pk and not hasattr(instance, "_no_signal") and instance.balance < 0:
+        from note.models import Note
+        old_note = Note.objects.get(pk=instance.pk)
+        if old_note.balance >= 0:
+            # Passage en négatif
+            instance.last_negative = timezone.now()
+            instance.send_mail_negative_balance()
+
+
 def delete_transaction(instance, **_kwargs):
     """
     Whenever we want to delete a transaction (caution with this), we ensure the transaction is invalid first.