path: root/railties/doc/guides/source/i18n.txt

The Rails Internationalization API
==================================

The Ruby I18n gem which is shipped with Ruby on Rails (starting from Rails 2.2) provides an easy-to-use and extensible framework for translating your application to a single custom language other than English or providing multi-language support in your application.

== How I18n in Ruby on Rails works

Internationalization is a complex problem. Natural languages differ in so many ways that it is hard to provide tools for solving all problems at once. For that reason the Rails I18n API focusses on:

* providing support for English and similar languages out of the box
* making it easy to customize and extend everything for other languages

=== The overall architecture of the library

To solve this the Ruby I18n gem is split into two parts:

* The public API which is just a Ruby module with a bunch of public methods and definitions how the library works.
* A shipped backend (which is intentionally named the Simple backend) that implements these methods.

As a user you should always only access the public methods on the I18n module but it is useful to know about the capabilities of the backend you use and maybe exchange the shipped Simple backend with a more powerful one.

=== The public I18n API

We will go into more detail about the public methods later but here's a quick overview. The most important methods are:

[source, ruby]
-------------------------------------------------------
translate         # lookup translations
localize          # localize Date and Time objects to local formats
-------------------------------------------------------

There are also attribute readers and writers for the following attributes:

[source, ruby]
-------------------------------------------------------
load_path         # announce your custom translation files
locale            # get and set the current locale
default_locale    # get and set the default locale
exception_handler # use a different exception_handler
backend           # use a different backend
-------------------------------------------------------

== Walkthrough: setup a simple I18n'ed Rails application

There are just a few, simple steps to get up and running with a I18n support for your application.

=== Configure the I18n module

First of all you want to tell the I18n library where it can find your custom translation files. You might also want to set your default locale to something else than English.

You can pick whatever directory and translation file naming scheme makes sense for you. The simplest thing possible is probably to put the following into an initializer:

[source, ruby]
-------------------------------------------------------
# in config/initializer/locale.rb

# tell the I18n library where to find your translations
I18n.load_path += Dir[ File.join(RAILS_ROOT, 'lib', 'locale', '*.{rb,yml}') ]

# you can omit this if you're happy with English as a default locale
I18n.default_locale = :"pt" 
-------------------------------------------------------

I18n.load_path is just a Ruby Array of paths to your translation files. The backend will lazy-load these translations when a translation is looked up for the first time. This makes it possible to just swap the backend with something else even after translations have already been announced.

=== Set the locale in each request

By default the I18n library will use the I18n.default_locale for looking up translations (if you do not specify a locale for a lookup) and this will, by default, en (English).

If you want to translate your Rails application to a single language other than English you can set I18n.default_locale to your locale. If you want to change the locale on a per-request basis though you can set it in a before_filter on the ApplicationController like this:

[source, ruby]
-------------------------------------------------------
before_filter :set_locale
def set_locale
  # if this is nil then I18n.default_locale will be used
  I18n.locale = params[:locale] 
end
-------------------------------------------------------

This will already work for URLs where you pass the locale as a query parameter as in example.com?locale=pt-BR (which is what Google also does). (TODO hints about other approaches in the resources section).

Now you've initialized I18n support for your application and told it which locale should be used. With that in place you're now ready for the really interesting stuff.

=== Internationalize your application

The process of "internationalization" usually means to abstract all strings and other locale specific bits out of your application (TODO reference to wikipedia). The process of "localization" means to then provide translations and localized formats for these bits.

So, let's internationalize something. You most probably have something like this in one of your applications:

[source, ruby]
-------------------------------------------------------
# config/routes.rb
ActionController::Routing::Routes.draw do |map|
  map.root :controller => 'home', :action => 'index'
end
  
# app/controllers/home_controller.rb
class HomeController < ApplicationController
  def index
    flash[:notice] = "Hello flash!"
  end
end

# app/views/home/index.html.erb
<h1>Hello world!</h1>
<p><%= flash[:notice] %></p>
-------------------------------------------------------

TODO screenshot

Obviously there are two strings that are localized to English. In order to internationalize this code replace these strings with calls to Rails' #t helper with a key that makes sense for the translation:

[source, ruby]
-------------------------------------------------------
# app/controllers/home_controller.rb
class HomeController < ApplicationController
  def index
    flash[:notice] = t(:hello_flash)
  end
end

# app/views/home/index.html.erb
<h1><%=t :hello_world %></h1>
<p><%= flash[:notice] %></p>
-------------------------------------------------------

TODO insert note about #t helper compared to I18n.t

TODO insert note/reference about structuring translation keys

When you now render this view it will show an error message that tells you that the translations for the keys :hello_world and :hello_flash are missing.

TODO screenshot

So let's add the missing translations (i.e. do the "localization" part):

[source, ruby]
-------------------------------------------------------
# lib/locale/en.yml
en-US:
  hello_world: Hello World
  hello_flash: Hello Flash
  
# lib/locale/pirate.yml
pirate:
  hello_world: Ahoy World
  hello_flash: Ahoy Flash
-------------------------------------------------------  

There you go. Your application now shows:

TODO screenshot

[source, ruby]
-------------------------------------------------------
I18n.t 'store.title'
I18n.l Time.now
-------------------------------------------------------


== Overview of the I18n API features

The following purposes are covered:

* lookup translations
* interpolate data into translations
* pluralize translations
* localize dates, numbers, currency etc.

=== Looking up translations

==== Basic lookup, scopes and nested keys

Translations are looked up by keys which can be both Symbols or Strings, so these calls are equivalent:

[source, ruby]
-------------------------------------------------------
I18n.t :message
I18n.t 'message'
-------------------------------------------------------

translate also takes a :scope option which can contain one or many additional keys that will be used to specify a “namespace” or scope for a translation key:

[source, ruby]
-------------------------------------------------------
I18n.t :invalid, :scope => [:active_record, :error_messages]
-------------------------------------------------------

This looks up the :invalid message in the ActiveRecord error messages.

Additionally, both the key and scopes can be specified as dot separated keys as in:

[source, ruby]
-------------------------------------------------------
I18n.translate :"active_record.error_messages.invalid"
-------------------------------------------------------

Thus the following calls are equivalent:

[source, ruby]
-------------------------------------------------------
I18n.t 'active_record.error_messages.invalid'
I18n.t 'error_messages.invalid', :scope => :active_record
I18n.t :invalid, :scope => 'active_record.error_messages'
I18n.t :invalid, :scope => [:active_record, :error_messages]
-------------------------------------------------------

==== Defaults

When a default option is given its value will be returned if the translation is missing:

[source, ruby]
-------------------------------------------------------
I18n.t :missing, :default => 'Not here'
# => 'Not here'
-------------------------------------------------------

If the default value is a Symbol it will be used as a key and translated. One can provide multiple values as default. The first one that results in a value will be returned.

E.g. the following first tries to translate the key :missing and then the key :also_missing. As both do not yield a result the string ‘Not here’ will be returned:

[source, ruby]
-------------------------------------------------------
I18n.t :missing, :default => [:also_missing, 'Not here']
# => 'Not here'
-------------------------------------------------------

==== Bulk and namespace lookup

To lookup multiple translations at once an array of keys can be passed:

[source, ruby]
-------------------------------------------------------
I18n.t [:odd, :even], :scope => 'active_record.error_messages'
# => ["must be odd", "must be even"]
-------------------------------------------------------

Also, a key can translate to a (potentially nested) hash as grouped translations. E.g. one can receive all ActiveRecord error messages as a Hash with:

[source, ruby]
-------------------------------------------------------
I18n.t 'active_record.error_messages'
# => { :inclusion => "is not included in the list", :exclusion => ... }
-------------------------------------------------------

=== Interpolation

TODO explain what this is good for

All options besides :default and :scope that are passed to #translate will be interpolated to the translation:

[source, ruby]
-------------------------------------------------------
I18n.backend.store_translations 'en', :thanks => 'Thanks {{name}}!'
I18n.translate :thanks, :name => 'Jeremy'
# => 'Thanks Jeremy!'
-------------------------------------------------------

If a translation uses :default or :scope as a interpolation variable an I18n::ReservedInterpolationKey exception is raised. If a translation expects an interpolation variable but it has not been passed to #translate an I18n::MissingInterpolationArgument exception is raised.

=== Pluralization

TODO explain what this is good for

The :count interpolation variable has a special role in that it both is interpolated to the translation and used to pick a pluralization from the translations according to the pluralization rules defined by CLDR:

[source, ruby]
-------------------------------------------------------
I18n.backend.store_translations 'en-US', :inbox => { # TODO change this
  :one => '1 message', 
  :other => '{{count}} messages'
}
I18n.translate :inbox, :count => 2
# => '2 messages'
-------------------------------------------------------

The algorithm for pluralizations in en-US is as simple as:

[source, ruby]
-------------------------------------------------------
entry[count == 1 ? 0 : 1]  
-------------------------------------------------------

I.e. the translation denoted as :one is regarded as singular, the other is used as plural (including the count being zero).

If the lookup for the key does not return an Hash suitable for pluralization an I18n::InvalidPluralizationData exception is raised.

=== Setting and passing a locale

The locale can be either set pseudo-globally to I18n.locale (which uses Thread.current like, e.g., Time.zone) or can be passed as an option to #translate and #localize.

If no locale is passed I18n.locale is used:

[source, ruby]
-------------------------------------------------------
I18n.locale = :'de'
I18n.t :foo 
I18n.l Time.now
-------------------------------------------------------

Explicitely passing a locale:

[source, ruby]
-------------------------------------------------------
I18n.t :foo, :locale => :'de'
I18n.l Time.now, :locale => :'de'
-------------------------------------------------------

I18n.locale defaults to I18n.default_locale which defaults to :'en'. The default locale can be set like this:

[source, ruby]
-------------------------------------------------------
I18n.default_locale = :'de'
-------------------------------------------------------

== How to store your custom translations

The shipped Simple backend allows you to store translations in both plain Ruby and YAML format. (2)

For example a Ruby Hash providing translations can look like this:

[source, ruby]
-------------------------------------------------------
{
  :'pt-BR' => {
    :foo => {
      :bar => "baz"
    }
  }
}
-------------------------------------------------------

The equivalent YAML file would look like this:

[source, ruby]
-------------------------------------------------------
"pt-BR":
  foo:
    bar: baz
-------------------------------------------------------    

As you see in both cases the toplevel key is the locale. :foo is a namespace key and :bar is the key for the translation "baz".

Here is a "real" example from the ActiveSupport en-US translations YAML file:

[source, ruby]
-------------------------------------------------------
"en":
  date:
    formats:
      default: "%Y-%m-%d"
      short: "%b %d"
      long: "%B %d, %Y"
-------------------------------------------------------

So, all of the following equivalent lookups will return the :short date format "%B %d":

[source, ruby]
-------------------------------------------------------
I18n.t 'date.formats.short'
I18n.t 'formats.short', :scope => :date
I18n.t :short, :scope => 'date.formats'
I18n.t :short, :scope => [:date, :formats]
-------------------------------------------------------

=== Translations for ActiveRecord models

You can use the methods Model.human_name and Model.human_attribute_name(attribute) to transparently lookup translations for your model and attribute names.

For example when you add the following translations:

en:
  activerecord:
    models:
      user: Dude
    attributes:
      user:
        login: "Handle"
      # will translate User attribute "login" as "Handle"
      
Then User.human_name will return "Dude" and User.human_attribute_name(:login) will return "Handle".
      
==== Error message scopes

ActiveRecord validation error messages can also be translated easily. ActiveRecord gives you a couple of namespaces where you can place your message translations in order to provide different messages and translation for certain models, attributes and/or validations. It also transparently takes single table inheritance into account.

This gives you quite powerful means to flexibly adjust your messages to your application's needs.

Consider a User model with a validates_presence_of validation for the name attribute like this:

[source, ruby]
-------------------------------------------------------
class User < ActiveRecord::Base
  validates_presence_of :name
end
-------------------------------------------------------

The key for the error message in this case is :blank. So ActiveRecord will first try to look up an error message with: 

[source, ruby]
-------------------------------------------------------
activerecord.errors.messages.models.user.attributes.name.blank
-------------------------------------------------------

If it's not there it will try:

[source, ruby]
-------------------------------------------------------
activerecord.errors.messages.models.user.blank
-------------------------------------------------------

If this is also not there it will use the default message from:

[source, ruby]
-------------------------------------------------------
activerecord.errors.messages.blank
-------------------------------------------------------

When your models are additionally using inheritance then the messages are looked up for the inherited model class names are looked up. 

For example, you might have an Admin model inheriting from User:

[source, ruby]
-------------------------------------------------------
class Admin < User
  validates_presence_of :name
end
-------------------------------------------------------

Then ActiveRecord will look for messages in this order:

[source, ruby]
-------------------------------------------------------
activerecord.errors.models.admin.attributes.title.blank
activerecord.errors.models.admin.blank
activerecord.errors.models.user.attributes.title.blank
activerecord.errors.models.user.blank
activerecord.errors.messages.blank
-------------------------------------------------------

This way you can provide special translations for various error messages at different points in your models inheritance chain and in the attributes, models or default scopes.

==== Error message interpolation

The translated model name and translated attribute name are always available for interpolation. 

So, for example, instead of the default error message "can not be blank" you could use the attribute name like this: "Please fill in your {{attribute}}".

Count and/or value are available where applicable. Count can be used for pluralization if present:

[grid="all"]
`---------------------------`----------------`---------------`----------------
validation                   with option      message         interpolation
validates_confirmation_of    -                :confirmation   -                 
validates_acceptance_of      -                :accepted       -                 
validates_presence_of        -                :blank          -                 
validates_length_of          :within, :in     :too_short      count             
validates_length_of          :within, :in     :too_long       count             
validates_length_of          :is              :wrong_length   count             
validates_length_of          :minimum         :too_short      count             
validates_length_of          :maximum         :too_long       count             
validates_uniqueness_of      -                :taken          value             
validates_format_of          -                :invalid        value             
validates_inclusion_of       -                :inclusion      value             
validates_exclusion_of       -                :exclusion      value             
validates_associated         -                :invalid        value             
validates_numericality_of    -                :not_a_number   value             
validates_numericality_of    :odd             :odd            value             
validates_numericality_of    :even            :even           value             
------------------------------------------------------------------------------


==== Translations for the ActiveRecord error_messages_for helper

If you are using the ActiveRecord error_messages_for helper you will want to add translations for it. 

Rails ships with the following translations:

[source, ruby]
-------------------------------------------------------
"en":
  activerecord:
    errors:
      template:
        header:
          one:   "1 error prohibited this {{model}} from being saved"
          other: "{{count}} errors prohibited this {{model}} from being saved"
        body: "There were problems with the following fields:"
-------------------------------------------------------


=== Other translations and localizations

Rails uses fixed strings and other localizations, such as format strings and other format information in a couple of helpers.

TODO list helpers and available keys

== Customize your I18n setup

=== Using different backends

For several reasons the shipped Simple backend only does the "simplest thing that ever could work" _for Ruby on Rails_ (1) ... which means that it is only guaranteed to work for English and, as a side effect, languages that are very similar to English. Also, the simple backend is only capable of reading translations but can not dynamically store them to any format.

That does not mean you're stuck with these limitations though. The Ruby I18n gem makes it very easy to exchange the Simple backend implementation with something else that fits better for your needs. E.g. you could exchange it with Globalize's Static backend:

[source, ruby]
-------------------------------------------------------
I18n.backend = Globalize::Backend::Static.new
-------------------------------------------------------

TODO expand this ...? list some backends and their features?

=== Using different exception handlers

TODO

* Explain what exceptions are raised and why we are using exceptions for communication from backend to frontend.
* Explain the default behaviour.
* Explain the :raise option

* Example 1: the Rails #t helper uses a custom exception handler that catches I18n::MissingTranslationData and wraps the message into a span with the CSS class "translation_missing"
* Example 2: for tests you might want a handler that just raises all exceptions all the time
* Example 3: a handler


== Resources

* http://rails-i18n.org

== Footnotes

(1) One of these reasons is that we don't want to any unnecessary load for applications that do not need any I18n capabilities, so we need to keep the I18n library as simple as possible for English. Another reason is that it is virtually impossible to implement a one-fits-all solution for all problems related to I18n for all existing languages. So a solution that allows us to exchange the entire implementation easily is appropriate anyway. This also makes it much easier to experiment with custom features and extensions.

(2) Other backends might allow or require to use other formats, e.g. a GetText backend might allow to read GetText files.

== Credits

== NOTES

How to contribute?