config-initializers-oauth.rb

# config/initializers/oauth.rb
Rails.application.config.middleware.use OmniAuth::Builder do
  provider :facebook,
           ENV['FACEBOOK_APP_ID'],
           ENV['FACEBOOK_SECRET_ID'],
           scope: 'email',
           info_fields: 'email',
           auth_type: 'rerequest'

  configure do |config|
    config.path_prefix = '/users/auth'
  end
end