omniauth_callbacks_controller.rb

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController

  def facebook
    oauth_callback("Facebook", "facebook_data")
  end

  def twitter
    oauth_callback("Twitter", "twitter_data")
  end

  def google_oauth2
    oauth_callback("Google+", "google_oauth2_data")
  end

  private

  def oauth_callback(notice_kind, session_key)
    @user = User.find_for_user_oauth(request.env["omniauth.auth"])

    if @user.persisted?
      flash[:notice] = I18n.t "devise.omniauth_callbacks.success", :kind => notice_kind
      sign_in_and_redirect @user, :event => :authentication
    else
      session["devise.#{session_key}"] = request.env["omniauth.auth"]
      redirect_to root_path
    end
  end
end

user.rb

def self.find_for_user_oauth(auth)
  user = User.where(email: auth.info.email).first
  // ...
    
end