Ensure IP subnets are unique

Note that currently, there is a bug in the admin: if you create multiple
subnets in the same IP pool in one go, then auto-allocation will fail.

coin/resources/migrations/0003_auto_20150203_1043.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+import netfields.fields
+import coin.resources.models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('resources', '0002_ipsubnet_name_server'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='ipsubnet',
+            name='inet',
+            field=netfields.fields.CidrAddressField(validators=[coin.resources.models.validate_subnet], max_length=43, blank=True, help_text='Laisser vide pour allouer automatiquement', unique=True, verbose_name='sous-r\xe9seau'),
+            preserve_default=True,
+        ),
+    ]

coin/resources/models.py

@@ -56,7 +56,7 @@ class IPPool(models.Model):
 
 class IPSubnet(models.Model):
     inet = CidrAddressField(blank=True, validators=[validate_subnet],
-                            verbose_name="sous-réseau",
+                            unique=True, verbose_name="sous-réseau",
                             help_text="Laisser vide pour allouer automatiquement")
     objects = NetManager()
     ip_pool = models.ForeignKey(IPPool, verbose_name="pool d'IP")