Improve set_locale

We discard session[:locale] as valid locale when it is no longer include in
the :available_locales

app/controllers/application_controller.rb

@@ -46,9 +46,11 @@ class ApplicationController < ActionController::Base
         session[:locale] = params[:locale]
       end
 
-      session[:locale] ||= I18n.default_locale
-
+      if session[:locale] && I18n.available_locales.include?(session[:locale].to_sym)
         locale = session[:locale]
+      else
+        locale = I18n.default_locale
+      end
 
       if current_user && current_user.locale != locale.to_s
         current_user.update(locale: locale)

spec/system/localization_spec.rb

@@ -88,4 +88,22 @@ describe "Localization" do
       end
     end
   end
+
+  scenario "uses default locale when session locale has disappeared" do
+    default_locales = I18n.available_locales
+
+    visit root_path(locale: :es)
+
+    expect(page).to have_content "Entrar"
+
+    begin
+      I18n.available_locales = default_locales - [:es]
+
+      visit root_path
+
+      expect(page).to have_content "Sign in"
+    ensure
+      I18n.available_locales = default_locales
+    end
+  end
 end