Monggomapper : Sort data with its Association Model's keys

This is basically a simple task, but I can't find a way how to perform a sort/order data in Mongomapper Model with keys that belongs from its association. Like suppose I have two models : class Product include MongoMapper::Document key :name, String key :product_category_id, ObjectId belongs_to :product_category end class ProductCategory include MongoMapper::Document key :name,