Rails 4.2.0 Engine RSpec main app routing

main_app_example.rb

# spec/support/main_app.rb
module MainAppRoute
  def main_app
    Rails.application.class.routes.url_helpers
  end
end

# In spec/rails_helper
RSpec.config do |config|
  config.include MainAppRoute
end

# Then in the controller specs
it 'redirects to main_app.root_path' do
  expect(response).to redirect_to main_app.root_path
end