Mobility

Mobility is a gem for storing and retrieving translations as attributes on a class. These translations could be the content of blog posts, captions on images, tags on bookmarks, or anything else you might want to store in different languages. For examples of what Mobility can do, see the Companies using Mobility section below.

Storage of translations is handled by customizable "backends" which encapsulate different storage strategies. The default way to store translations is to put them all in a set of two shared tables, but many alternatives are also supported, including translatable columns and model translation tables, as well as database-specific storage solutions such as json/jsonb and Hstore (for PostgreSQL).

Mobility is a cross-platform solution, currently supporting both ActiveRecord and Sequel ORM, with support for other platforms planned.

For a detailed introduction to Mobility, see Translating with Mobility. See also my talk at RubyConf 2018, Building Generic Software, where I explain the thinking behind Mobility's design.

If you're coming from Globalize, be sure to also read the Migrating from Globalize section of the wiki.

Installation

Add this line to your application's Gemfile:

gem 'mobility', '~> 0.8.6'

Mobility is cryptographically signed. To be sure the gem you install hasn't been tampered with, add my public key as a trusted certificate and install:

gem cert --add <(curl -Ls https://raw.github.com/shioyama/mobility/master/certs/shioyama.pem)
gem install mobility -P MediumSecurity

The MediumSecurity trust profile will verify signed gems, but allow the installation of unsigned dependencies.

ActiveRecord (Rails)

Requirements:

ActiveRecord >= 5.0

(Support for most backends and features is also supported with ActiveRecord/Rails 4.2, but there are some tests still failing. To see exactly what might not work, check pending specs in Rails 4.2 Travis builds.)

To translate attributes on a model, extend Mobility, then call translates passing in one or more attributes as well as a hash of options (see below).

If using Mobility in a Rails project, you can run the generator to create an initializer and a migration to create shared translation tables for the default KeyValue backend:

rails generate mobility:install

(If you do not plan to use the default backend, you may want to use the --without_tables option here to skip the migration generation.)

The generator will create an initializer file config/initializers/mobility.rb with the lines:

Mobility.configure do |config|
  config.default_backend = :key_value
  config.accessor_method = :translates
  config.query_method    = :i18n
end

To use a different default backend, set default_backend to another value (see possibilities below).

You will likely also want to set default values for the various translation options described below. You can set these defaults by assigning values to keys on the config.default_options hash. Below, we turn on the Dirty plugin by default, so it will be enabled for all models.

You can also set defaults for backend-specific options. Below, we set the default type option for the KeyValue backend to :string (this is unnecessary and will be ignored if you are using a different backend).

 Mobility.configure do |config|
   config.default_backend = :key_value
   config.accessor_method = :translates
   config.query_method    = :i18n
+  config.default_options[:dirty] = true
+  config.default_options[:type]  = :string
end

We will assume the configuration above in the examples that follow. Other configuration options are described in the API docs.

See Getting Started to get started translating your models.

Sequel

Requirements:

Sequel >= 4.0

You can extend Mobility just like in ActiveRecord, or you can use the mobility plugin, which does the same thing:

class Word < ::Sequel::Model
  plugin :mobility
  translates :name, :meaning
end

Otherwise everything is (almost) identical to AR, with the exception that there is no equivalent to a Rails generator (so you will need to create the migration for any translation table(s) yourself, using Rails generators as a reference).

The models in examples below all inherit from ApplicationRecord, but everything works exactly the same if the parent class is Sequel::Model.

Usage

Getting Started

Once the install generator has been run to generate translation tables, using Mobility is as easy as adding a few lines to any class you want to translate. Simply pass one or more attribute names to the translates method with a hash of options, like this:

class Word < ApplicationRecord
  extend Mobility
  translates :name, :meaning
end

Note: When using the KeyValue backend, use the options hash to pass each attribute's type:

class Word < ApplicationRecord
  extend Mobility
  translates :name,    type: :string
  translates :meaning, type: :text
end

This is important because this is how Mobility knows to which of the two translation tables it should save your translation.

You now have translated attributes name and meaning on the model Word. You can set their values like you would any other attribute:

word = Word.new
word.name = "mobility"
word.meaning = "(noun): quality of being changeable, adaptable or versatile"
word.name
#=> "mobility"
word.meaning
#=> "(noun): quality of being changeable, adaptable or versatile"
word.save
word = Word.first
word.name
#=> "mobility"
word.meaning
#=> "(noun): quality of being changeable, adaptable or versatile"

Presence methods are also supported:

word.name?
#=> true
word.name = nil
word.name?
#=> false
word.name = ""
word.name?
#=> false

What's different here is that the value of these attributes changes with the value of I18n.locale:

I18n.locale = :ja
word.name
#=> nil
word.meaning
#=> nil

The name and meaning of this word are not defined in any locale except English. Let's define them in Japanese and save the model:

word.name = "モビリティ"
word.meaning = "(名詞):動きやすさ、可動性"
word.name
#=> "モビリティ"
word.meaning
#=> "(名詞):動きやすさ、可動性"
word.save

Now our word has names and meanings in two different languages:

word = Word.first
I18n.locale = :en
word.name
#=> "mobility"
word.meaning
#=> "(noun): quality of being changeable, adaptable or versatile"
I18n.locale = :ja
word.name
#=> "モビリティ"
word.meaning
#=> "(名詞):動きやすさ、可動性"

Internally, Mobility is mapping the values in different locales to storage locations, usually database columns. By default these values are stored as keys (attribute names) and values (attribute translations) on a set of translation tables, one for strings and one for text columns, but this can be easily changed and/or customized (see the Backends section below).

Getting and Setting Translations

The easiest way to get or set a translation is to use the getter and setter methods described above (word.name and word.name=), but you may want to access the value of an attribute in a specific locale, independent of the current value of I18n.locale (or Mobility.locale). There are a few ways to do this.

The first way is to define locale-specific methods, one for each locale you want to access directly on a given attribute. These are called "locale accessors" in Mobility, and they can be defined by passing a locale_accessors option when defining translated attributes on the model class:

class Word < ApplicationRecord
  extend Mobility
  translates :name, locale_accessors: [:en, :ja]
end

Since we have enabled locale accessors for English and Japanese, we can access translations for these locales with name_en and name_ja:

word.name_en
#=> "mobility"
word.name_ja
#=> "モビリティ"
word.name_en = "foo"
word.name
#=> "foo"

Other locales, however, will not work:

word.name_ru
#=> NoMethodError: undefined method `name_ru' for #<Word id: ... >

To generate methods for all locales in I18n.available_locales (at the time the model is first loaded), use locale_accessors: true.

An alternative to using the locale_accessors option is to use the fallthrough_accessors option, with fallthrough_accessors: true. This uses Ruby's method_missing method to implicitly define the same methods as above, but supporting any locale without any method definitions. (Locale accessors and fallthrough locales can be used together without conflict, with locale accessors taking precedence if defined for a given locale.)

For example, if we define Word this way:

class Word < ApplicationRecord
  extend Mobility
  translates :name, fallthrough_accessors: true
end

... then we can access any locale we want, without specifying them upfront:

word = Word.new
word.name_fr = "mobilité"
word.name_fr
#=> "mobilité"
word.name_ja = "モビリティ"
word.name_ja
#=> "モビリティ"

(Note however that Mobility will complain if you have I18n.enforce_available_locales set to true and you try accessing a locale not present in I18n.available_locales; set it to false if you want to allow any locale.)

Another way to fetch values in a locale is to pass the locale option to the getter method, like this:

word.name(locale: :en)
#=> "mobility"
word.name(locale: :fr)
#=> "mobilité"

Note that setting the locale this way will pass an option locale: true to the backend and all plugins. Plugins may use this option to change their behavior (passing the locale explicitly this way, for example, disables fallbacks, see below for details).

You can also set the value of an attribute this way; however, since the word.name = <value> syntax does not accept any options, the only way to do this is to use send (this is included mostly for consistency):

word.send(:name=, "mobiliteit", locale: :nl)
word.name_nl
#=> "mobiliteit"

Yet another way to get and set translated attributes is to call read and write on the storage backend, which can be accessed using the method <attribute>_backend. Without worrying too much about the details of how this works for now, the syntax for doing this is simple:

word.name_backend.read(:en)
#=> "mobility"
word.name_backend.read(:nl)
#=> "mobiliteit"
word.name_backend.write(:en, "foo")
word.name_backend.read(:en)
#=> "foo"

Internally, all methods for accessing translated attributes ultimately end up reading and writing from the backend instance this way. (The write methods do not call underlying backend's methods to persist the change. This is up to the user, so e.g. with ActiveRecord you should call save write the changes to the database).

Note that accessor methods are defined in an included module, so you can wrap reads or writes in custom logic:

class Post < ApplicationRecord
  extend Mobility
  translates :title

  def title(*)
    super.reverse
  end
end

Setting the Locale

It may not always be desirable to use I18n.locale to set the locale for content translations. For example, a user whose interface is in English (I18n.locale is :en) may want to see content in Japanese. If you use I18n.locale exclusively for the locale, you will have a hard time showing stored translations in one language while showing the interface in another language.

For these cases, Mobility also has its own locale, which defaults to I18n.locale but can be set independently:

I18n.locale = :en
Mobility.locale              #=> :en
Mobility.locale = :fr
Mobility.locale              #=> :fr
I18n.locale                  #=> :en

To set the Mobility locale in a block, you can use Mobility.with_locale (like I18n.with_locale):

Mobility.locale = :en
Mobility.with_locale(:ja) do
  Mobility.locale            #=> :ja
end
Mobility.locale              #=> :en

Mobility uses RequestStore to reset these global variables after every request, so you don't need to worry about thread safety. If you're not using Rails, consult RequestStore's README for details on how to configure it for your use case.

Fallbacks

Mobility offers basic support for translation fallbacks. To enable fallbacks, pass a hash with fallbacks for each locale as an option when defining translated attributes on a class:

class Word < ApplicationRecord
  extend Mobility
  translates :name,    fallbacks: { de: :ja, fr: :ja }, locale_accessors: true
  translates :meaning, fallbacks: { de: :ja, fr: :ja }, locale_accessors: true
end

Internally, Mobility assigns the fallbacks hash to an instance of I18n::Locale::Fallbacks.new (this can be customized by setting the fallbacks_generator configuration option, see the API documentation on configuration).

By setting fallbacks for German and French to Japanese, values will fall through to the Japanese value if none is present for either of these locales, but not for other locales:

Mobility.locale = :ja
word = Word.create(name: "モビリティ", meaning: "(名詞):動きやすさ、可動性")
Mobility.locale = :de
word.name
#=> "モビリティ"
word.meaning
#=> "(名詞):動きやすさ、可動性"
Mobility.locale = :fr
word.name
#=> "モビリティ"
word.meaning
#=> "(名詞):動きやすさ、可動性"
Mobility.locale = :ru
word.name
#=> nil
word.meaning
#=> nil

You can optionally disable fallbacks to get the real value for a given locale (for example, to check if a value in a particular locale is set or not) by passing fallback: false (singular, not plural) to the getter method:

Mobility.locale = :de
word.meaning(fallback: false)
#=> nil
Mobility.locale = :fr
word.meaning(fallback: false)
#=> nil
Mobility.locale = :ja
word.meaning(fallback: false)
#=> "(名詞):動きやすさ、可動性"

You can also set the fallback locales for a single read by passing one or more locales:

Mobility.with_locale(:fr) do
  word.meaning = "(nf): aptitude à bouger, à se déplacer, à changer, à évoluer"
end
word.save
Mobility.locale = :de
word.meaning(fallback: false)
#=> nil
word.meaning(fallback: :fr)
#=> "(nf): aptitude à bouger, à se déplacer, à changer, à évoluer"
word.meaning(fallback: [:ja, :fr])
#=> "(名詞):動きやすさ、可動性"

Also note that passing a locale option into an attribute reader or writer, or using locale accessors or fallthrough accessors to get or set any attribute value, will disable fallbacks (just like fallback: false). (This will take precedence over any value of the fallback option.)

Continuing from the last example:

word.meaning(locale: :de)
#=> nil
word.meaning_de
#=> nil
Mobility.with_locale(:de) { word.meaning }
#=> "(名詞):動きやすさ、可動性"

For more details, see the API documentation on fallbacks and this article on I18n fallbacks.

Default values

Another option is to assign a default value, which will be used if the result of a fetch would otherwise be nil:

class Word < ApplicationRecord
  extend Mobility
  translates :name, default: 'foo'
end

Mobility.locale = :ja
word = Word.create(name: "モビリティ")
word.name
#=> "モビリティ"
Mobility.locale = :de
word.name
#=> "foo"

You can override the default by passing a default option to the attribute reader:

word.name
#=> 'foo'
word.name(default: nil)
#=> nil
word.name(default: 'bar')
#=> 'bar'

The default can also be a Proc, which will be called with the context as the model itself, and passed optional arguments (attribute, locale and options passed to accessor) which can be used to customize behaviour. See the API docs for details.

Dirty Tracking

Dirty tracking (tracking of changed attributes) can be enabled for models which support it. Currently this is models which include ActiveModel::Dirty (like ActiveRecord::Base) and Sequel models (through the dirty plugin).

Enabling dirty tracking is as simple as sending the dirty: true option when defining a translated attribute. The way dirty tracking works is somewhat dependent on the model class (ActiveModel or Sequel); we will describe the ActiveModel implementation here.

First, enable dirty tracking (note that this is a persisted AR model, although dirty tracking is not specific to AR and works for non-persisted models as well):

class Post < ApplicationRecord
  extend Mobility
  translates :title, dirty: true
end

(If you want to enable dirty tracking on all models, set the config.default_options[:dirty] option in your Mobility configuration.)

Let's assume we start with a post with a title in English and Japanese:

post = Post.create(title: "Introducing Mobility")
Mobility.with_locale(:ja) { post.title = "モビリティの紹介" }
post.save

Now let's change the title:

post = Post.first
post.title                      #=> "Introducing Mobility"
post.title = "a new title"
Mobility.with_locale(:ja) do
  post.title                    #=> "モビリティの紹介"
  post.title = "新しいタイトル"
  post.title                    #=> "新しいタイトル"
end

Now you can use dirty methods as you would any other (untranslated) attribute:

post.title_was
#=> "Introducing Mobility"
Mobility.locale = :ja
post.title_was
#=> "モビリティの紹介"
post.changed
["title_en", "title_ja"]
post.save

You can also access previous_changes:

post.previous_changes
#=>
{
  "title_en" =>
    [
      "Introducing Mobility",
      "a new title"
    ],
  "title_ja" =>
    [
      "モビリティの紹介",
      "新しいタイトル"
    ]
}

Notice that Mobility uses locale suffixes to indicate which locale has changed; dirty tracking is implemented this way to ensure that it is clear what has changed in which locale, avoiding any possible ambiguity.

For performance reasons, it is highly recommended that when using the Dirty plugin, you also enable locale accessors for all locales which will be used, so that methods like title_en above are defined; otherwise they will be caught by method_missing (using fallthrough accessors), which is much slower. The easiest way to do this is to set config.default_options[:locale_accessors] = true in your Mobility config, and make sure that I18n.available_locales includes all locales you use in production.

For more details on dirty tracking, see the API documentation.

Cache

The Mobility cache caches localized values that have been fetched once so they can be quickly retrieved again. The cache is enabled by default and should generally only be disabled when debugging; this can be done by passing cache: false when defining an attribute, like this:

class Word < ApplicationRecord
  extend Mobility
  translates :name, cache: false
end

You can also turn off the cache for a single fetch by passing cache: false to the getter method, i.e. post.title(cache: false).

The cache is normally just a hash with locale keys and string (translation) values, but some backends (e.g. KeyValue and Table backends) have slightly more complex implementations.

Querying

Mobility backends also support querying on translated attributes, in two different ways. The first is via query methods like where (and not and find_by in ActiveRecord, and except in Sequel). To query this way, use the i18n class method, which will return a model relation or dataset extended with Mobility-specific query method overrides.

So for ActiveRecord, assuming a model using KeyValue as its default backend:

class Post < ApplicationRecord
  extend Mobility
  translates :title,   type: :string
  translates :content, type: :text
end

... we can query for posts with title "foo" and content "bar" just as we would query on untranslated attributes, and Mobility will convert the queries to whatever the backend requires to actually return the correct results:

Post.i18n.find_by(title: "foo", content: "bar")

results in the SQL:

SELECT "posts".* FROM "posts"
INNER JOIN "mobility_string_translations" "Post_title_en_string_translations"
  ON "Post_title_en_string_translations"."key" = 'title'
  AND "Post_title_en_string_translations"."locale" = 'en'
  AND "Post_title_en_string_translations"."translatable_type" = 'Post'
  AND "Post_title_en_string_translations"."translatable_id" = "posts"."id"
INNER JOIN "mobility_text_translations" "Post_content_en_text_translations"
  ON "Post_content_en_text_translations"."key" = 'content'
  AND "Post_content_en_text_translations"."locale" = 'en'
  AND "Post_content_en_text_translations"."translatable_type" = 'Post'
  AND "Post_content_en_text_translations"."translatable_id" = "posts"."id"
WHERE "Post_title_en_string_translations"."value" = 'foo'
  AND "Post_content_en_text_translations"."value" = 'bar'

As can be seen in the query above, behind the scenes Mobility joins two tables, one with string translations and one with text translations, and aliases the joins for each attribute so as to match the particular model, attribute(s), locale(s) and value(s) passed in to the query. Details of how this is done can be found in the Wiki page for the KeyValue backend.

You can also use methods like order, select, pluck and group on translated attributes just as you would with normal attributes, and Mobility will handle generating the appropriate SQL:

Post.i18n.pluck(:title)
#=> ["foo", "bar", ...]

If you would prefer to avoid the i18n scope everywhere, you can define it as a default scope on your model:

class Post < ApplicationRecord
  extend Mobility
  translates :title,   type: :string
  translates :content, type: :text
  default_scope { i18n }
end

Now translated attributes can be queried just like normal attributes:

Post.find_by(title: "Introducing Mobility")
#=> finds post with English title "Introducing Mobility"

If you want more fine-grained control over your queries, you can alternatively pass a block to the query method and call attribute names from the block scope to build Arel predicates:

Post.i18n do
  title.matches("foo").and(content.matches("bar"))
end

which generates the same SQL as above, except the WHERE clause becomes:

SELECT "posts".* FROM "posts"
  ...
WHERE "Post_title_en_string_translations"."value" ILIKE 'foo'
  AND "Post_content_en_text_translations"."value" ILIKE 'bar'

The block-format query format is very powerful and allows you to build complex backend-independent queries on translated and untranslated attributes without having to deal with the details of how these translations are stored. The same interface is supported with Sequel to build datasets.

Backends

Mobility supports different storage strategies, called "backends". The default backend is the KeyValue backend, which stores translations in two tables, by default named mobility_text_translations and mobility_string_translations.

You can set the default backend to a different value in the global configuration, or you can set it explicitly when defining a translated attribute, like this:

class Word < ApplicationRecord
  translates :name, backend: :table
end

This would set the name attribute to use the Table backend (see below). The type option (type: :string or type: :text) is missing here because this is an option specific to the KeyValue backend (specifying which shared table to store translations on). Backends have their own specific options; see the Wiki and API documentation for which options are available for each.

Everything else described above (fallbacks, dirty tracking, locale accessors, caching, querying, etc) is the same regardless of which backend you use.

Table Backend (like Globalize)

The Table backend stores translations as columns on a model-specific table. If your model uses the table posts, then by default this backend will store an attribute title on a table post_translations, and join the table to retrieve the translated value.

To use the table backend on a model, you will need to first create a translation table for the model, which (with Rails) you can do using the mobility:translations generator:

rails generate mobility:translations post title:string content:text

This will generate the post_translations table with columns title and content, and all other necessary columns and indices. For more details see the Table Backend page of the wiki and API documentation on the Mobility::Backend::Table class.

Column Backend (like Traco)

The Column backend stores translations as columns with locale suffixes on the model table. For an attribute title, these would be of the form title_en, title_fr, etc.

Use the mobility:translations generator to add columns for locales in I18n.available_locales to your model:

rails generate mobility:translations post title:string content:text

For more details, see the Column Backend page of the wiki and API documentation on the Mobility::Backend::Column class.

PostgreSQL-specific Backends

Mobility also supports JSON and Hstore storage options, if you are using PostgreSQL as your database. To use this option, create column(s) on the model table for each translated attribute, and set your backend to :json, :jsonb or :hstore. If you are using Sequel, note that you will need to enable the pg_json or pg_hstore extensions with DB.extension :pg_json or DB.extension :pg_hstore (where DB is your database instance).

Another option is to store all your translations on a single jsonb column (one per model). This is called the "container" backend.

For details on these backends, see the Postgres Backend and Container Backend pages of the wiki and in the API documentation (Mobility::Backend::Jsonb and Mobility::Backend::Hstore).

Note: The Json backend (:json) may also work with recent versions of MySQL with JSON column support, although this backend/db combination is not tested. See this issue for details.

Development

Custom Backends

Although Mobility is primarily oriented toward storing ActiveRecord model translations, it can potentially be used to handle storing translations in other formats. In particular, the features mentioned above (locale accessors, caching, fallbacks, dirty tracking to some degree) are not specific to database storage.

To use a custom backend, simply pass the name of a class which includes Mobility::Backend to translates:

class MyBackend
  include Mobility::Backend
  # ...
end

class MyClass
  extend Mobility
  translates :foo, backend: MyBackend
end

For details on how to define a backend class, see the Introduction to Mobility Backends page of the wiki and the API documentation on the Mobility::Backend module.

Testing Backends

All included backends are tested against a suite of shared specs which ensure they conform to the same expected behaviour. These examples can be found in:

spec/support/shared_examples/accessor_examples.rb (minimal specs testing translation setting/getting)
spec/support/shared_examples/querying_examples.rb (specs for querying)
spec/support/shared_examples/serialization_examples.rb (specialized specs for backends which store translations as a Hash: serialized, hstore, json and jsonb backends)

A minimal test can simply define a model class and use helpers defined in spec/support/helpers.rb to run these examples, by extending either Helpers::ActiveRecord or Helpers::Sequel:

describe MyBackend do
  extend Helpers::ActiveRecord

  before do
    stub_const 'MyPost', Class.new(ActiveRecord::Base)
    MyPost.extend Mobility
    MyPost.translates :title, :content, backend: MyBackend
  end

  include_accessor_examples 'MyPost'
  include_querying_examples 'MyPost'
  # ...
end

Shared examples expect the model class to have translated attributes title and content, and an untranslated boolean column published. These defaults can be changed, see the shared examples for details.

Backends are also each tested against specialized specs targeted at their particular implementations.

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Mobility

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Getting and Setting Translations

Setting the Locale

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Querying

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Mobility

Installation

ActiveRecord (Rails)

Sequel

Usage

Getting Started

Getting and Setting Translations

Setting the Locale

Fallbacks

Default values

Dirty Tracking

Cache

Querying

Backends

Table Backend (like Globalize)

Column Backend (like Traco)

PostgreSQL-specific Backends

Development

Custom Backends

Testing Backends

Integrations

Tutorials

More Information

Companies using Mobility

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