public function associate($rowId, $model)
    {
        if(is_string($model) && ! class_exists($model)) {
            throw new UnknownModelException("The supplied model {$model} does not exist.");
        }

        $cartItem = $this->get($rowId);

        $cartItem->associate($model);

        $content = $this->getContent();

        $content->put($cartItem->rowId, $cartItem);

        $this->session->put($this->instance, $content);
    }