Allow to delete a member without deleting its services invoices

Invoices must be kept for six years…

Ref #30 Ref #137

coin/billing/migrations/0011_auto_20180819_0221.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('billing', '0010_new_billing_system_data'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='invoicedetail',
+            name='offersubscription',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.SET_NULL, default=None, blank=True, to='offers.OfferSubscription', null=True, verbose_name='abonnement'),
+        ),
+    ]

coin/billing/models.py

@@ -326,6 +326,7 @@ class InvoiceDetail(models.Model):
                                 related_name='details')
     offersubscription = models.ForeignKey(OfferSubscription, null=True,
                                           blank=True, default=None,
+                                          on_delete=models.SET_NULL,
                                           verbose_name='abonnement')
     period_from = models.DateField(
         default=start_of_month,