topic name should be unique *per site*

proposals/migrations/0004_auto_20160711_2058.py

@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.7 on 2016-07-11 20:58
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('proposals', '0003_auto_20160711_1933'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='topic',
+            name='name',
+            field=models.CharField(max_length=128, verbose_name='Name'),
+        ),
+        migrations.AlterUniqueTogether(
+            name='topic',
+            unique_together=set([('site', 'name')]),
+        ),
+    ]

proposals/models.py

@@ -18,11 +18,14 @@ class Topic(PonyConfModel):
 
     site = models.ForeignKey(Site, on_delete=models.CASCADE)
 
-    name = models.CharField(max_length=128, verbose_name='Name', unique=True)
+    name = models.CharField(max_length=128, verbose_name='Name')
     slug = AutoSlugField(populate_from='name', unique=True)
 
     reviewers = models.ManyToManyField(User, blank=True)
 
+    class Meta:
+        unique_together = ('site', 'name')
+
     def __str__(self):
         return self.name