Abhishek Srivastava

If you are using https://github.com/zquestz/omniauth-google-oauth2 make sure to follow the note in README.
"You must enable the "Contacts API" and "Google+ API" via the Google API console."

This fixed the 'Could not authenticate you from GoogleOauth2 because "Invalid credentials".' problem for me.

@james - my bad, it seems i got confused as to where i should put the conf file - http://stackoverflow.com/a/19384824/753705

At the last step, when trying to restart nginx getting this error:
nginx: [emerg] unknown directive "upstream" in /etc/nginx/nginx.conf:1 ; nginx: configuration file /etc/nginx/nginx.conf test failed" when trying to restart nginx

Question asked here - http://stackoverflow.com/questions/19349057/nginx-unknown-directive-upstream

Fixed the issue by doing a Javascipt redirect

<script>
  top.location="https://www.facebook.com/dialog/oauth?client_id=FACEBOOK_APP_ID&redirect_uri=http://apps.facebook.com/app-namespace";
</script>

Looks like facebook does not allow redirect within iframe (and canvas apps are in iframe)

I have updated my 'config/application.rb' as suggested in the this tip.
But no matter what I do I keep getting this error in my console while trying to do a redirect and the oauth dialog does not appear -

Refused to display https://www.facebook.com/dialog/oauth?client_id=FACEBOOK_APP_ID&redirect_uri=http://localhost:3000' in a frame because it set 'X-Frame-Options' to 'DENY'.