Ruby on Rails允许跟踪/索引受保护的页面

I need to allow indexing of user protected pages for Facebook sharing. Basically, I have a RoR app that offers coupons and I need for users to be able to share the coupon on facebook. Only problem is that a user must have an account to view all the coupon details on each coupon's page. I want them to be able to share the coupon (title, price only) with a link to that coupon, but the link to the coupon will essentially redirect to the sign up page if the user does not have an account. Therefore, when Facebook (and other indexing robots) go to pull info from the coupon URL, they also get redirected to the sign up page and index the sign up page info instead of the coupon page info.I need to allow indexing of user protected page