使用omniauth时的controller，这与devise里的omniauthable模块差不多。但实现手法又有一点不同，选择哪个，看情况吧。

authentications_controller.rb

class AuthenticationsController < Devise::OmniauthCallbacksController
  before_action :create
  
  def github
  end
  
  def create
    puts "=== omniauth is: #{request.env["omniauth"]} ==="
    puts "=== omniauth.auth is: #{request.env["omniauth.auth"]} ==="

    omniauth = request.env["omniauth.auth"]
    authentication = Authentication.find_by_provider_and_uid(omniauth['provider'], omniauth['uid'])
    if authentication
      flash[:notice] = "成功登录."
      sign_in_and_redirect(:user, authentication.user)
    elsif current_user
      current_user.authentications.create(
        :provider => omniauth['provider'],
        :uid => omniauth['uid'] ,
        :token => omniauth['credentials']['token'],
        :secret => omniauth['credentials']['secret']
      )
      redirect_to root_url, notice: "Authentication successful."
    else
      puts "=== will User.new"
      user = User.new
      user.authentications.build(
        :provider => omniauth['provider'],
        :uid => omniauth['uid'],
        :token => omniauth['credentials']['token'],
        :secret => omniauth['credentials']['secret']
      )
      # 没有意义的开始
      user.email = omniauth[:info][:email]
      user.password = SecureRandom.hex(10)
      # 没有意义的结束
      if user.save
        flash[:notice] = "成功登录. You password is: #{user.try(:password)}"
        sign_in_and_redirect(:user, user)
      else
        puts "=== User.new and falture"
        puts "=== user.errors #{user.errors.to_json}"
        session[:omniauth] = omniauth.except('extra')
        redirect_to new_user_registration_url
      end
    end
  end
end