Devise + OmniAuth in engine

routes.rb

MyEngine::Engine.routes.draw do

  devise_scope :user do 
    providers = Regexp.union(Devise.omniauth_providers.map(&:to_s))
    match 'users/auth/:provider', 
      constraints: { provider: providers },
      to: 'omniauth_callbacks#passthru', 
      as: :omniauth_authorize, 
      via: [:get, :post]

    match 'users/auth/:action/callback', 
      constraints: { action: providers },
      to: 'omniauth_callbacks', 
      as: :omniauth_callback, 
      via: [:get, :post]
  end

  devise_for  :users, class_name: 'MyEngine::User', module: :devise
end