diff -r cd5e8fb64b74 -r 56850f5c73f6 src/notes/models/core.py
--- a/src/notes/models/core.py	Tue Jul 10 15:18:03 2018 +0200
+++ b/src/notes/models/core.py	Wed Jul 18 17:32:09 2018 +0200
@@ -38,7 +38,8 @@
     group = models.ForeignKey(
         Group,
         null=False,
-        verbose_name=_('Session|group')
+        verbose_name=_('Session|group'),
+        on_delete=models.PROTECT
     )
     protocol = models.TextField(
         null=True,