class Recommender(object):

    def get_product_key(self, id):
        return 'product:{}:purchased_with'.format(id)

    def products_bought(self, products):
        product_ids = [p.id for p in products]
        for product_id in product_ids:
            for with_id in product_ids:
                if product_id != with_id:
                    r.zincrby(self.get_product_key(product_id),
                              with_id,
                              amount=1)