Make i18n fallbacks configurable and fallback to the default locale by default...

Make i18n fallbacks configurable and fallback to the default locale by default in production [#4428 state:resolved]

Allows to configure locale fallbacks through config.i18n.fallbacks. The default setting
config.i18n.fallbacks = true in production.rb will make I18n.t lookup fallback to the
I18n.default_locale if a translation could not be found for the current or given locale.

config.fallbacks = true

config.fallbacks.map = { :ca => :es }
config.fallbacks.defaults = [:'es-ES', :es]

config.fallbacks = [:'es-ES', :es]

config.fallbacks = { :ca => :es }

config.fallbacks = [:'es-ES', :es, { :ca => :es }]
Signed-off-by: NPratik Naik <pratiknaik@gmail.com>

activesupport/lib/active_support/railtie.rb

@@ -33,6 +33,7 @@ class Railtie < Rails::Railtie
     config.i18n = ActiveSupport::OrderedOptions.new
     config.i18n.railties_load_path = []
     config.i18n.load_path = []
+    config.i18n.fallbacks = ActiveSupport::OrderedOptions.new
 
     initializer "i18n.initialize" do
       ActiveSupport.on_load(:i18n) do
@@ -53,6 +54,8 @@ class Railtie < Rails::Railtie
           app.config.i18n.load_path.unshift(*value)
         when :load_path
           I18n.load_path += value
+        when :fallbacks
+          init_fallbacks(value) if value && validate_fallbacks(value)
         else
           I18n.send("#{setting}=", value)
         end
@@ -60,5 +63,37 @@ class Railtie < Rails::Railtie
 
       I18n.reload!
     end
+
+    class << self
+      protected
+
+        def init_fallbacks(fallbacks)
+          include_fallbacks_module
+          args = case fallbacks
+          when ActiveSupport::OrderedOptions
+            [*(fallbacks[:defaults] || []) << fallbacks[:map]].compact
+          when Hash, Array
+            Array.wrap(fallbacks)
+          else # TrueClass
+            []
+          end
+          I18n.fallbacks = I18n::Locale::Fallbacks.new(*args)
+        end
+
+        def include_fallbacks_module
+          I18n.backend.class.send(:include, I18n::Backend::Fallbacks)
+        end
+      
+        def validate_fallbacks(fallbacks)
+          case fallbacks
+          when ActiveSupport::OrderedOptions
+            !fallbacks.empty?
+          when TrueClass, Array, Hash
+            true
+          else
+            raise "Unexpected fallback type #{fallbacks.inspect}"
+          end
+        end
+    end
   end
 end
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railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt

@@ -39,4 +39,8 @@
 
   # Enable threaded mode
   # config.threadsafe!
+
+  # Enable locale fallbacks for I18n (makes lookups for any locale fall back to
+  # the I18n.default_locale when a translation can not be found)
+  config.i18n.fallbacks = true
 end

railties/test/railties/i18n_railtie_test.rb

+require "isolation/abstract_unit"
+
+module RailtiesTest
+  class I18nRailtieTest < Test::Unit::TestCase
+    include ActiveSupport::Testing::Isolation
+
+    def setup
+      build_app
+      boot_rails
+      FileUtils.rm_rf("#{app_path}/config/environments")
+      require "rails/all"
+      @old_path = I18n.load_path
+    end
+
+    def teardown
+      I18n.load_path = @old_path || []
+      I18n.backend = nil
+    end
+
+    def load_app
+      require "#{app_path}/config/environment"
+    end
+
+    def assert_fallbacks(fallbacks)
+      fallbacks.each do |locale, expected|
+        actual = I18n.fallbacks[locale]
+        assert_equal expected, actual, "expected fallbacks for #{locale.inspect} to be #{expected.inspect}, but were #{actual.inspect}"
+      end
+    end
+
+    def assert_no_fallbacks
+      assert !I18n.backend.class.included_modules.include?(I18n::Backend::Fallbacks)
+    end
+
+    test "config.i18n.load_path gets added to I18n.load_path" do
+      I18n.load_path = ['existing/path/to/locales']
+      I18n::Railtie.config.i18n.load_path = ['new/path/to/locales']
+      load_app
+
+      assert I18n.load_path.include?('existing/path/to/locales')
+      assert I18n.load_path.include?('new/path/to/locales')
+    end
+
+    test "not using config.i18n.fallbacks does not initialize I18n.fallbacks" do
+      I18n.backend = Class.new { include I18n::Backend::Base }.new # can't uninclude modules, so use a tmp backend class
+      load_app
+      assert_no_fallbacks
+    end
+
+    test "config.i18n.fallbacks = true initializes I18n.fallbacks with default settings" do
+      I18n::Railtie.config.i18n.fallbacks = true
+      load_app
+      assert_fallbacks :de => [:de, :en]
+    end
+
+    test "config.i18n.fallbacks.defaults = [:'en-US'] initializes fallbacks with en-US as a fallback default" do
+      I18n::Railtie.config.i18n.fallbacks.defaults = [:'en-US']
+      load_app
+      assert_fallbacks :de => [:de, :'en-US', :en]
+    end
+
+    test "config.i18n.fallbacks.map = { :ca => :'es-ES' } initializes fallbacks with a mapping ca => es-ES" do
+      I18n::Railtie.config.i18n.fallbacks.map = { :ca => :'es-ES' }
+      load_app
+      assert_fallbacks :ca => [:ca, :"es-ES", :es, :en]
+    end
+
+    test "[shortcut] config.i18n.fallbacks = [:'en-US'] initializes fallbacks with en-US as a fallback default" do
+      I18n::Railtie.config.i18n.fallbacks = [:'en-US']
+      load_app
+      assert_fallbacks :de => [:de, :'en-US', :en]
+    end
+
+    test "[shortcut] config.i18n.fallbacks = [{ :ca => :'es-ES' }] initializes fallbacks with a mapping de-AT => de-DE" do
+      I18n::Railtie.config.i18n.fallbacks.map = { :ca => :'es-ES' }
+      load_app
+      assert_fallbacks :ca => [:ca, :"es-ES", :es, :en]
+    end
+
+    test "[shortcut] config.i18n.fallbacks = [:'en-US', { :ca => :'es-ES' }] initializes fallbacks with the given arguments" do
+      I18n::Railtie.config.i18n.fallbacks = [:'en-US', { :ca => :'es-ES' }]
+      load_app
+      assert_fallbacks :ca => [:ca, :"es-ES", :es, :'en-US', :en]
+    end
+  end
+end
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