Add primary key `id` to `category_featured_topics` and `topic_users`

db/migrate/20130712163509_add_missing_id_columns.rb

@@ -0,0 +1,11 @@
+class AddMissingIdColumns < ActiveRecord::Migration
+  def up
+    add_column :category_featured_topics, :id, :primary_key
+    add_column :topic_users, :id, :primary_key
+  end
+
+  def down
+    remove_column :category_featured_topics, :id
+    remove_column :topic_users, :id
+  end
+end