Get current user id without access_token for poll app?

Can I get the user_id without him acepting my app?

I need to build a poll app that should not require auth do vote. If I can store the user_id I can make sure all votes are unique...

Thanks

Solution

It is very limited what you can get when the user didn't install your app. Because getting the uid is the same as getting all public data for the user (you can query all public data with that uid and any access_token of some other user). So, unfortunately, you have to ask for an app install when you want to use Facebook to track if the user has already voted.