fallbacks.rb

# I18n locale fallbacks are useful when you want your application to use
# translations from other locales when translations for the current locale are
# missing. E.g. you might want to use :en translations when translations in
# your applications main locale :de are missing.
#
# To enable locale fallbacks you can simply include the Fallbacks module to
# the Simple backend - or whatever other backend you are using:
#
#   I18n::Backend::Simple.include(I18n::Backend::Fallbacks)
module I18n
  @@fallbacks = nil

  class << self
    # Returns the current fallbacks implementation. Defaults to +I18n::Locale::Fallbacks+.
    def fallbacks
      @@fallbacks ||= I18n::Locale::Fallbacks.new
    end

    # Sets the current fallbacks implementation. Use this to set a different fallbacks implementation.
    def fallbacks=(fallbacks)
      @@fallbacks = fallbacks
    end
  end

  module Backend
    module Fallbacks
      # Overwrites the Base backend translate method so that it will try each
      # locale given by I18n.fallbacks for the given locale. E.g. for the
      # locale :"de-DE" it might try the locales :"de-DE", :de and :en
      # (depends on the fallbacks implementation) until it finds a result with
      # the given options. If it does not find any result for any of the
      # locales it will then throw MissingTranslation as usual.
      #
      # The default option takes precedence over fallback locales only when
      # it's a Symbol. When the default contains a String, Proc or Hash
      # it is evaluated last after all the fallback locales have been tried.
      def translate(locale, key, options = {})
        return super unless options.fetch(:fallback, true)
        return super if options[:fallback_in_progress]
        original_default = options[:default]
        extract_non_symbol_default!(options)

        begin
          options[:fallback_in_progress] = true
          I18n.fallbacks[locale].each do |fallback|
            begin
              catch(:exception) do
                result = super(fallback, key, options)
                return result unless result.nil?
              end
            rescue I18n::InvalidLocale
              # we do nothing when the locale is invalid, as this is a fallback anyways.
            end
          end
        ensure
          options.delete(:fallback_in_progress)
        end

        return super(locale, nil, options.merge(:default => original_default)) if options.key?(:default)
        throw(:exception, I18n::MissingTranslation.new(locale, key, options))
      end

      def extract_non_symbol_default!(options)
        defaults = [options[:default]].flatten
        first_non_symbol_default = defaults.detect{|default| !default.is_a?(Symbol)}
        if first_non_symbol_default
          options[:default] = defaults[0, defaults.index(first_non_symbol_default)]
        end
        return first_non_symbol_default
      end

      def exists?(locale, key)
        I18n.fallbacks[locale].each do |fallback|
          begin
            return true if super(fallback, key)
          rescue I18n::InvalidLocale
            # we do nothing when the locale is invalid, as this is a fallback anyways.
          end
        end

        false
      end
    end
  end
end