experimental cascading backend (thanks to Clemens Kofler for the init…

…ial idea)

lib/i18n/backend.rb

@@ -3,6 +3,7 @@ module Backend
     autoload :ActiveRecord,  'i18n/backend/active_record'
     autoload :Base,          'i18n/backend/base'
     autoload :Cache,         'i18n/backend/cache'
+    autoload :Cascade,       'i18n/backend/cascade'
     autoload :Chain,         'i18n/backend/chain'
     autoload :Fallbacks,     'i18n/backend/fallbacks'
     autoload :Gettext,       'i18n/backend/gettext'

lib/i18n/backend/cascade.rb

@@ -0,0 +1,20 @@
+# encoding: utf-8
+
+module I18n
+  @@fallbacks = nil
+
+  module Backend
+    module Cascade
+      def lookup(locale, key, scope = [], separator = nil)
+        return unless key
+        locale, *scope = I18n.send(:normalize_translation_keys, locale, key, scope, separator)
+        key = scope.pop
+
+        begin
+          result = super
+          return result unless result.nil?
+        end while scope.pop
+      end
+    end
+  end
+end

test/cases/backend/cascade_test.rb

@@ -0,0 +1,66 @@
+# encoding: utf-8
+
+require File.expand_path(File.dirname(__FILE__) + '/../../test_helper')
+
+class I18nBackendCascadeTest < Test::Unit::TestCase
+  class Backend
+    include I18n::Backend::Base
+    include I18n::Backend::Cascade
+  end
+
+  def setup
+    I18n.backend = Backend.new
+    store_translations(:en,
+      :foo => 'foo',
+      :bar => { :baz => 'baz' }
+    )
+  end
+
+  define_method "test: still returns an existing translation as usual" do
+    assert_equal 'foo', I18n.t(:foo)
+    assert_equal 'baz', I18n.t(:'bar.baz')
+  end
+
+  define_method "test: falls back by cutting keys off the end of the scope" do
+    assert_equal 'foo', I18n.t(:'does_not_exist.foo')
+    assert_equal 'foo', I18n.t(:'does_not_exist.does_not_exist.foo')
+
+    assert_equal 'baz', I18n.t(:'bar.does_not_exist.baz')
+    assert_equal 'baz', I18n.t(:'bar.does_not_exist.does_not_exist.baz')
+  end
+
+  define_method "test: raises I18n::MissingTranslationData exception when no translation was found" do
+    assert_raises(I18n::MissingTranslationData) { I18n.t(:'foo.does_not_exist', :raise => true) }
+    assert_raises(I18n::MissingTranslationData) { I18n.t(:'bar.baz.does_not_exist', :raise => true) }
+    assert_raises(I18n::MissingTranslationData) { I18n.t(:'does_not_exist.bar.baz', :raise => true) }
+  end
+
+  define_method "test: cascades before evaluating the default" do
+    assert_equal 'foo', I18n.t(:foo, :scope => :does_not_exist, :default => 'default')
+  end
+
+  define_method "test: let's us assemble required fallbacks for ActiveRecord validation messages" do
+    store_translations(:en,
+      :errors => {
+        :reply => {
+          :title => {
+            :blank => 'blank on reply title'
+          },
+          :taken => 'taken on reply'
+        },
+        :topic => {
+          :title => {
+            :format => 'format on topic title'
+          },
+          :length => 'length on topic'
+        },
+        :odd => 'odd on errors'
+      }
+    )
+    assert_equal 'blank on reply title',  I18n.t(:'errors.reply.title.blank',  :default => :'errors.topic.title.blank')
+    assert_equal 'taken on reply',        I18n.t(:'errors.reply.title.taken',  :default => :'errors.topic.title.taken')
+    assert_equal 'format on topic title', I18n.t(:'errors.reply.title.format', :default => :'errors.topic.title.format')
+    assert_equal 'length on topic',       I18n.t(:'errors.reply.title.length', :default => :'errors.topic.title.length')
+    assert_equal 'odd on errors',         I18n.t(:'errors.reply.title.odd',    :default => :'errors.topic.title.odd')
+  end
+end