protect base object before removing through removing service(_env) or…

… conf. class/module (allegro#2985)

src/ralph/assets/migrations/0024_auto_20170322_1148.py

@@ -0,0 +1,30 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('assets', '0023_category_allow_deployment'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='baseobject',
+            name='configuration_path',
+            field=models.ForeignKey(verbose_name='configuration path', blank=True, help_text='path to configuration for this object, for example path to puppet class', on_delete=django.db.models.deletion.PROTECT, null=True, to='assets.ConfigurationClass'),
+        ),
+        migrations.AlterField(
+            model_name='baseobject',
+            name='parent',
+            field=models.ForeignKey(related_name='children', to='assets.BaseObject', blank=True, on_delete=django.db.models.deletion.SET_NULL, null=True),
+        ),
+        migrations.AlterField(
+            model_name='baseobject',
+            name='service_env',
+            field=models.ForeignKey(to='assets.ServiceEnvironment', on_delete=django.db.models.deletion.PROTECT, null=True),
+        ),
+    ]

src/ralph/assets/models/base.py

@@ -69,10 +69,13 @@ class BaseObject(
     """Base object mixin."""
     # TODO: dynamically limit parent basing on model
     parent = models.ForeignKey(
-        'self', null=True, blank=True, related_name='children'
+        'self', null=True, blank=True, related_name='children',
+        on_delete=models.SET_NULL
     )
     remarks = models.TextField(blank=True)
-    service_env = models.ForeignKey('ServiceEnvironment', null=True)
+    service_env = models.ForeignKey(
+        'ServiceEnvironment', null=True, on_delete=models.PROTECT
+    )
 
     @property
     def _str_with_type(self):
@@ -89,7 +92,8 @@ def _str_with_type(self):
         help_text=_(
             'path to configuration for this object, for example path to puppet '
             'class'
-        )
+        ),
+        on_delete=models.PROTECT
     )
 
     @property