FIX: destroy CategoryGroup when Category or Group is destroyed

db/migrate/20140831191346_remove_category_groups_orphaned_by_removing_category_or_group.rb

@@ -0,0 +1,13 @@
+class RemoveCategoryGroupsOrphanedByRemovingCategoryOrGroup < ActiveRecord::Migration
+  def up
+    execute "DELETE FROM category_groups
+             WHERE group_id NOT IN (
+                 SELECT groups.id FROM groups)
+              OR category_id NOT IN (
+                 SELECT categories.id FROM categories)"
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end