如何使用Codeigniter和Tank Auth保持会话数据

I'm using Codeigniter and Tank Auth for a e-commerce website. I'm finding that if a customer puts items into the cart (using the Codeigniter built in cart class) and then registers on the site the session is lost along with their cart (I'm presuming it generates a new session for some reason instead of keeping the same session).I'm using Codeigniter and Tank Auth for a e-com