adds omniauth basic authentication process with Twitter, including an intermediate step to ask the user for her email if not provided by the OAuth provider - Twitter, for instance

app/controllers/users/omniauth_callbacks_controller.rb

@@ -0,0 +1,30 @@
+class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
+  def self.provides_callback_for(provider)
+    class_eval %Q{
+      def #{provider}
+        @user = User.find_for_oauth(env["omniauth.auth"], current_user)
+
+        if @user.persisted?
+          sign_in_and_redirect @user, event: :authentication
+          set_flash_message(:notice, :success, kind: "#{provider}".capitalize) if is_navigational_format?
+        else
+          session["devise.#{provider}_data"] = env["omniauth.auth"]
+          redirect_to new_user_registration_url
+        end
+      end
+    }
+  end
+
+  # [:twitter, :facebook, :google_oauth2].each do |provider|
+  [:twitter].each do |provider|
+    provides_callback_for provider
+  end
+
+  def after_sign_in_path_for(resource)
+    if resource.email_provided?
+      super(resource)
+    else
+      finish_signup_path
+    end
+  end
+end