Don't delete network if gateway address is removed (allegro#2959)

src/ralph/networks/migrations/0010_auto_20170216_1230.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 = [
+        ('networks', '0009_auto_20160823_0921'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='network',
+            name='gateway',
+            field=models.ForeignKey(to='networks.IPAddress', blank=True, on_delete=django.db.models.deletion.SET_NULL, verbose_name='Gateway address', null=True, related_name='gateway_network'),
+        ),
+    ]

src/ralph/networks/models/networks.py

@@ -234,7 +234,7 @@ class Network(
     address._filter_title = _('Network Class')
     gateway = models.ForeignKey(
         'IPAddress', verbose_name=_('Gateway address'), null=True, blank=True,
-        related_name='gateway_network'
+        related_name='gateway_network', on_delete=models.SET_NULL,
     )
     remarks = models.TextField(
         verbose_name=_('remarks'),