Subdomain accessing only specific features with Rails

Question

I’m using Rails 4.2.2

I have the following code in the config/Routes.Rb file:

Rails.application.routes.draw do
    namespace :api, constraints: { subdomain: 'api' }, path: '/' do
        resources :users
    end

    root "pages#home"

    resources :users
    resources :posts
end

Here’s what’s going on:

Visiting the domain dominio.com I can’t access the API, so far so good.

The problem arises when visiting api.dominio.com, wanted only access to the resource users which is in the namespace :api and nothing else. But the opposite happens. even using the subdomain api.dominio.com i can access root and other resources outside it like the :posts.