places_api.rb

# app/controllers/api/v1/cities/places_api.rb
 
class Api::V1::Cities::PlacesAPI < Api::V1::BaseAPI
  namespace 'cities/:city_id' do
    resource :places do
      helpers ::Api::V1::CityHelper
 
      params do
        requires :city_id, :type => Integer
      end

      get '/' do
        places = resource_city.places
        present :places, places, :with => ::Api::V1::City::PlaceEntity
      end
    end
  end
end

v1.rb

# app/controllers/api/v1.rb
 
class Api::V1 < Grape::API
  format :json
  content_type :json, 'application/json; charset=utf-8'
 
  version 'v1'
 
  mount CitiesAPI
  mount Cities::PlacesAPI
end