OAuth "Invalid verification code format" - ruby-on-rails

Question

I've got a production app and occasionally I see this error appear in the logs.. I can't seem to duplicate it on my end, so it seems like there is a user out there somewhere with some specific settings / cookies / etc that are causing this problem. I am not sure... The log shows that this user was redirected to this callback url: "users/auth/facebook/callback?code=AQCayaAoFOruFgwbfg1D682j8DbxOt0CZYNH3Vv5RtYKlQgSzISyN8ygTn25W_RTl3fu35cS1-tl5ArZ9B_XylwORP0hGU6st8P6TyTYUzfiR1m0poaSRkX-KBeWiBvT6IUsm-Af0VJcUNTQPg-dM1F9y5CgJ2bTJEJqhCE9wYlvkUY3kguwcl3TQ48FTT4-PhA///"
The actual error is:
RuntimeError: #<OAuth2::Response:0x1da7fae0 #error=#<OAuth2::Error: OAuth2::Error>, #options={:parse=>:query}, #parsed={"{\"error\":{\"message\":\"Invalid verification code format.\",\"type\":\"OAuthException\"}}"=>nil}, #response=#<Faraday::Response:0x1da8fa80 #on_complete_callbacks=[], #env={:response=>#<Faraday::Response:0x1da8fa80 ...>, :request_headers=>{"Content-Type"=>"application/x-www-form-urlencoded"}, :body=>"{\"error\":{\"message\":\"Invalid verification code format.\",\"type\":\"OAuthException\"}}", :status=>400, :url=>#<Addressable::URI:0xed52bf0 URI:https://graph.facebook.com/oauth/access_token>, :request=>{:proxy=>nil}, :parallel_manager=>nil, :response_headers=>{"expires"=>"Sat, 01 Jan 2000 00:00:00 GMT", "access-control-allow-origin"=>"*", "content-type"=>"text/javascript; charset=UTF-8", "connection"=>"close", "www-authenticate"=>"OAuth \"Facebook Platform\" \"invalid_code\" \"Invalid verification code format.\"", "date"=>"Thu, 13 Oct 2011 15:58:29 GMT", "content-length"=>"81", "cache-control"=>"no-store", "x-fb-rev"=>"457598", "x-fb-server"=>"10.65.13.60", "pragma"=>"no-cache"}, :ssl=>{:ca_file=>"/etc/pki/tls/certs/ca-bundle.crt"}, :method=>:post}>>
and the parameters were:
{"code"=> "AQCayaAoFOruFgwbfg1D682j8DbxOt0CZYNH3Vv5RtYKlQgSzISyN8ygTn25W_RTl3fu35cS1-tl5ArZ9B_XylwORP0hGU6st8P6TyTYUzfiR1m0poaSRkX-KBeWiBvT6IUsm-Af0VJcUNTQPg-dM1F9y5CgJ2bTJEJqhCE9wYlvkUY3kguwcl3TQ48FTT4-PhA///",
"action"=>"",
"controller"=>""}
I know if I manually go to "users/auth/facebook/callback?code=blah" it will trigger this same error because the callback code parameter is obviously bogus, but it doesn't look like there's anything suspicious about the actual code in the user's params hash-- so I am wondering why it would be an invalid format?
Has anyone else experienced this?

Answer 1

I think the following link provides the answer: http://developers.facebook.com/docs/authentication/#authenticating-users-in-a-web-application
You first need to call https://graph.facebook.com/oauth/authorize with your client_id and redirect_uri. This will then redirect you back to the redirect_uri, with a verification code in the query string that you can pass to your https://graph.facebook.com/oauth/access_token call (via the code parameter) to exchange for an oAuth access token.
Good luck! :)