Set catalog null on deletion

rdmo/projects/migrations/0016_catalog_on_delete.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.1 on 2018-06-01 08:33
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('projects', '0015_permissions'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='project',
+            name='catalog',
+            field=models.ForeignKey(help_text='The catalog which will be used for this project.', null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='+', to='questions.Catalog', verbose_name='Catalog'),
+        ),
+    ]

rdmo/projects/models.py

@@ -40,7 +40,7 @@ class Project(Model):
         help_text=_('A description for this project (optional).')
     )
     catalog = models.ForeignKey(
-        Catalog, related_name='+',
+        Catalog, related_name='+', on_delete=models.SET_NULL, null=True,
         verbose_name=_('Catalog'),
         help_text=_('The catalog which will be used for this project.')
     )