[#1633] Migration removing unused keywords

Keywords that aren't used in any project are deleted.

akvo/rsr/migrations/0025_auto_20150820_0144.py

@@ -0,0 +1,32 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+def remove_unused_keys(apps, schema_editor):
+    Keyword = apps.get_model('rsr', 'Keyword')
+    Project = apps.get_model('rsr', 'Project')
+    print
+
+    for keyword in Keyword.objects.all():
+        projects = Project.objects.filter(keywords__exact=keyword)
+        if not projects:
+            print "Deleting keyword {}".format(keyword.label)
+            keyword.delete()
+
+    print "Keyword distribution after migration:"
+    for keyword in Keyword.objects.all():
+        projects = Project.objects.filter(keywords__exact=keyword)
+        print keyword.label, projects.count()
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('rsr', '0024_auto_20150819_1149'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            remove_unused_keys
+        ),
+    ]