[prefix_delegation] Add on_delete to DelegatedReverseDns.delegated_prefix

prefix_delegation/models.py

@@ -105,8 +105,14 @@ class Nameserver(models.Model):
 
 class DelegatedReverseDns(models.Model):
 
-    delegated_prefix = models.OneToOneField(DelegatedPrefix, related_name='delegated_reverse_dns')
-    nameservers = models.ManyToManyField(Nameserver)
+    delegated_prefix = models.OneToOneField(
+        DelegatedPrefix,
+        on_delete=models.CASCADE,
+        related_name='delegated_reverse_dns'
+    )
+    nameservers = models.ManyToManyField(
+        Nameserver
+    )
 
     def owner(self):
         return self.delegated_prefix.owner