Make facebook the ONLY / default login for devise canvas app

If your facebook app is asking for user permissions, and they have authorized it, the signed_request parameter that is passed to your rails app should contain the details you need. So what you can do is decode the parameter, and grab the facebook user id from the hash itself.

Try something like this:

class ApplicationController    before_filter :inspect_signed_request  protected   def base64_url_decode(input)     input += ('=' * (4 - input.length % 4) % 4)     Base64.decode64(input.tr('-_', '+/'))   end       def inspect_signed_request     encoded_sig, payload = params[:signed_request].split('.', 2)      # decode the data     sig = base64_url_decode(encoded_sig)     data = JSON.parse(self.class.base64_url_decode(payload))     raise data.inspect   end end 

That will basically raise an exception, showing you the details of the signed request. Essentially, you can just parse it out as such, and check for the signed request FB ID value. If it's not there, you can add some code to redirect them to the login form instead.

Source: http://facebook.stackoverflow.com/questions/13032571/make-facebook-the-only-default-login-for-devise-canvas-app

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