http://www.eisbahn.jp/yoichiro/2011/10/oauth2-0_google-api.html
http://railscasts.com/episodes/304-omniauth-identity?language=ja&view=asciicast
providerとして:google_oauth2がloadされない.devise+omniauth+omniauth-google-oauth2だとうまく動かないらしい./omniauth/google-oauth2.rbをコピーして、google.rbを作成した.
↓
rake routesすると
Received wrong number of arguments. ["ffjpXU4qu_Tk38YeUOTFKma8", {:access_token_path=>"/accounts/OAuthGetAccessToken", :authorize_path=>"/accounts/OAuthAuthorizeToken", :request_token_path=>"/accounts/OAuthGetRequestToken", :site=>"https://www.google.com"}]
というエラーに変わった.つぎ.