omniauth.rb

module OmniAuth
  module Strategies
    class Facebook
      def client
        # https://developers.facebook.com/apps にてアプリを登録
        #
        # apps = {
        #   'サイトURL' => {id: 'App ID', secret: 'App Secret'}
        # }
        apps = {
          'http://www.example.com/' => {id: 'AAAAAAAAAAAAAAA',
                                        secret: 'BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB'},
          'http://www.example.net/' => {id: 'CCCCCCCCCCCCCCC',
                                        secret: 'DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD'}
        }

        if (app = apps["#{request.base_url}/"])
          options.client_id = app[:id]
          options.client_secret = app[:secret]
        end

        super
      end
    end
  end
end

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :facebook, ENV['FACEBOOK_KEY'], ENV['FACEBOOK_SECRET']
end