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InstallationAndUsage02  

Release-0.2
Updated Jun 3, 2011 by eschler

Release: 0.2

Modeltranslation

The modeltranslation application can be used to translate existing models to an arbitrary number of languages without having to change the original model classes. It uses a registration approach (comparable to Django's admin app) to be able to add translations to existing or new projects and is fully integrated into the Django admin backend.

Contents

  • Features
  • Installation
    • Configure the project's settings.py
    • Registering models and their fields for translation
    • Changes automatically applied to the model class
  • Accessing translated and translation fields
    • Rules for translated field access
    • Examples for translated field access
  • Django admin backend integration
    • Tweaks applied to the admin
    • TranslationAdmin in combination with other admin classes
    • Inlines
  • The update_translation_fields command
  • Caveats
    • Accessing translated fields outside views
  • Related projects
    • django-multilingual
    • django-multilingual-model
    • transdb
    • i18ndynamic
    • django-pluggable-model-i18n

Features

  • Unlimited number of target languages
  • Add translations without changing existing models
  • Django admin support
  • Supports inherited models

Installation

To install the application please follow these steps. Each step is described in detail in the following sections:

  1. Add the modeltranslation app to the INSTALLED_APPS variable of your project's settings.py.
  2. Configure your languages in the settings.py.
  3. Create a translation.py in your project directory and register TranslationOptions for every model you want to translate.
  4. Configure the TRANSLATION_REGISTRY variable in your settings.py.
  5. Sync the database using manage.py syncdb (note that this only applies if the models registered in the translations.py did not have been synced to the database before. If they did - read further down what to do in that case.

Configure the project's settings.py

The following variables have to be added to or edited in the project's settings.py:

settings.INSTALLED_APPS

Make sure that the modeltranslation app is listed in your INSTALLED_APPS variable:

INSTALLED_APPS = (
    ...
    'modeltranslation',
    ....
)

Important: Also make sure that the app can be found on a path contained in your PYTHONPATH environment variable.

settings.LANGUAGES

The LANGUAGES variable must contain all languages used for translation. The first language is treated as the default language.

The modeltranslation application uses the list of languages to add localized fields to the models registered for translation. To use the languages de and en in your project, set the settings.LANGUAGES variable like this (where de is the default language):

gettext = lambda s: s
LANGUAGES = (
    ('de', gettext('German')),
    ('en', gettext('English')),
)

Note that the gettext lambda function is not a feature of the modeltranslation app, but rather required for Django to be able to (statically) translate the verbose names of the languages using the standard i18n solution.

settings.TRANSLATION_REGISTRY

In order to be able to import the project's translation.py registration file the TRANSLATION_REGISTRY must be set to a value in the form <PROJECT_MODULE>.translation. E.g. if your project is located in a folder named myproject the TRANSLATION_REGISTRY must be set like this:

TRANSLATION_REGISTRY = "myproject.translation"

Registering models and their fields for translation

The modeltranslation app can translate CharField and TextFieldbased fields of any model class. For each model to translate a translation option class containg the fields to translate is registered with the modeltranslation app.

Registering models and their fields for translation requires the following steps:

  1. Create a translation.py in your project directory.
  2. Create a translation option class for every model to translate.
  3. Register the model and the translation option class at the modeltranslation.translator.translator

The modeltranslation application reads the translation.py file in your project directory thereby triggering the registration of the translation options found in the file.

A translation option is a class that declares which fields of a model to translate. The class must derive from modeltranslation.ModelTranslationand it must provide a fields attribute storing the list of fieldnames. The option class must be registered with the modeltranslation.translator.translator instance.

Note: In contrast to the Django admin application which looks for admin.py files in the project and application directories, the modeltranslation app looks only for one translation.py file in the project directory.

To illustrate this let's have a look at a simple example using a News model. The news in this example only contains a title and a text field. Instead of a news, this could be any Django model class:

class News(models.Model):
    title = models.CharField(max_length=255)
    text = models.TextField()

In order to tell the modeltranslation app to translate the title and text field, create a translation.py file in your project directory and add the following:

from modeltranslation.translator import translator, TranslationOptions
from some.news.models import News

class NewsTranslationOptions(TranslationOptions):
    fields = ('title', 'text',)

translator.register(News, NewsTranslationOptions)

Note that this does not require to change the News model in any way, it's only imported. The NewsTranslationOptions derives from TranslationOptions and provides the fields attribute. Finally the model and it's translation options are registered at the translator object.

At this point you are mostly done and the model classes registered for translation will have been added some auto-magical fields. The next section explains how things are working under the hood.

Changes automatically applied to the model class

After registering the News model for transaltion an SQL dump of the News app will look like this:

$ ./manage.py sqlall news
BEGIN;
CREATE TABLE `news_news` (
    `id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
    `title` varchar(255) NOT NULL,
    `title_de` varchar(255) NULL,
    `title_en` varchar(255) NULL,
    `text` longtext NULL,
    `text_de` longtext NULL,
    `text_en` longtext NULL,
)
;
ALTER TABLE `news_news` ADD CONSTRAINT page_id_refs_id_3edd1f0d FOREIGN KEY (`page_id`) REFERENCES `page_page` (`id`);
CREATE INDEX `news_news_page_id` ON `news_news` (`page_id`);
COMMIT;

Note the title_de, title_en, text_de and text_en fields which are not declared in the original News model class but rather have been added by the modeltranslation app. These are called translation fields. There will be one for every language in your project's settings.py.

The name of these additional fields is build using the original name of the translated field and appending one of the language identifiers found in the settings.LANGUAGES.

As these fields are added to the registered model class as fully valid Django model fields, they will appear in the db schema for the model although it has not been specified on the model explicitly.

If you are starting a fresh project and have considered your translation needs in the beginning then simply sync your database and you are ready to use the translated models.

In case you are translating an existing project and your models have already been synced to the database you will need to alter the tables in your database and add these additional translation fields. Note that all added fields are declared null=True not matter if the original field is required. In other words - all translations are optional. To populate the default translation fields added by the modeltranslation application you can use the update_translation_fields command below. See the The update_translation_fields command section for more infos on this.

Accessing translated and translation fields

The modeltranslation app changes the behaviour of the translated fields. To explain this consider the News example again. The original News model looked like this:

class News(models.Model):
    title = models.CharField(max_length=255)
    text = models.TextField()

Now that it is registered with the modeltranslation app the model looks like this - note the additional fields automatically added by the app:

class News(models.Model):
    title = models.CharField(max_length=255)  # original/translated field
    title_de = models.CharField(null=True, blank=True, max_length=255)  # default translation field
    title_en = models.CharField(null=True, blank=True, max_length=255)  # translation field
    text = models.TextField() # original/translated field
    text_de = models.TextField(null=True, blank=True) # default translation field
    text_en = models.TextField(null=True, blank=True) # translation field

The example above assumes that the default language is de, therefore the title_de and text_de fields are marked as the default translation fields. If the default language is en, the title_en and text_enfields would be the default translation fields.

Rules for translated field access

So now when it comes to setting and getting the value of the original and the translation fields the following rules apply:

Rule 1

Reading the value from the original field returns the value translated to the current language.

Rule 2

Assigning a value to the original field also updates the value in the associated default translation field.

Rule 3

Assigning a value to the default translation field also updates the original field - note that the value of the original field will not be updated until the model instance is saved.

Rule 4

If both fields - the original and the default translation field - are updated at the same time, the default translation field wins.

Examples for translated field access

Because the whole point of using the modeltranslation app is translating dynamic content, the fields marked for translation are somehow special when it comes to accessing them. The value returned by a translated field is depending on the current language setting. "Language setting" is referring to the Django set_language view and the corresponding get_lang function.

Assuming the current language is de in the News example from above, the translated title field will return the value from the title_de field:

# Assuming the current language is "de"
n = News.objects.all()[0]
t = n.title # returns german translation

# Assuming the current language is "en"
t = n.title # returns english translation

This feature is implemented using Python descriptors making it happen without the need to touch the original model classes in any way. The descriptor uses the django.utils.i18n.get_language function to determine the current language.

Django admin backend integration

In order to be able to edit the translations via the admin backend you need to register a special admin class for the translated models. The admin class must derive from modeltranslation.admin.TranslationAdmin which does some funky patching on all your models registered for translation:

from django.contrib import admin
from modeltranslation.admin import TranslationAdmin

class NewsAdmin(TranslationAdmin):
    list_display = ('title',)

admin.site.register(News, NewsAdmin)

Tweaks applied to the admin

The TranslationAdmin class does only implement one special method which is def formfield_for_dbfield(self, db_field, **kwargs). This method does the following:

  1. Removes the original field from every admin form by setting it editable=False.
  2. Copies the widget of the original field to each of it's translation fields.
  3. Checks if the - now removed - original field was required and if so makes the default translation field required instead.

TranslationAdmin in combination with other admin classes

If there already exists a custom admin class for a translated model and you don't want or can't edit that class directly there is another solution.

Taken the News example let's say there is a NewsAdmin class defined by the News app itself. This app is not yours or you don't want to touch it at all, but it has this nice admin class:

class NewsAdmin(model.Admin):
    def formfield_for_dbfield(self, db_field, **kwargs):
        # does some funky stuff with the formfield here

So a first attempt might be to create your own admin class which subclasses NewsAdmin and TranslationAdmin to combine stuff like so:

class MyTranslatedNewsAdmin(NewsAdmin, TranslationAdmin):
    pass

Unfortunately this won't work because Python can only execute one of the formfield_for_dbfield methods. Since both admin class implement this method Python must make a decision and it chooses the first class NewsAdmin. The functionality from TranslationAdmin will not be executed and translation in the admin will not work for this class.

But don't panic, here's a solution:

class MyTranslatedNewsAdmin(NewsAdmin, TranslationAdmin):
    def formfield_for_dbfield(self, db_field, **kwargs):
        field = super(MyTranslatedNewsAdmin, self).formfield_for_dbfield(db_field, **kwargs)
        self.patch_translation_field(db_field, field, **kwargs)
        return field

This implements the formfield_for_dbfield such that both functionalities will be executed. The first line calls the superclass method which in this case will be the one of NewsAdmin because it is the first class inherited from. The TranslationAdmin capsulates all it's functionality in the patch_translation_field(db_field, field, **kwargs) method and the formfield_for_dbfield implementation of the TranslationAdmin class simply calls it. You can copy this behaviour by calling it from a custom admin class and that's done in the example above. After that the field is fully patched for translation and finally returned.

Inlines

New in development version: Support for tabular and stacked inlines, common and generic ones.

A translated inline must derive from one of the following classes:

  • modeltranslation.admin.TranslationTabularInline
  • modeltranslation.admin.TranslationStackedInline
  • modeltranslation.admin.TranslationGenericTabularInline
  • modeltranslation.admin.TranslationGenericStackedInline

Just like TranslationAdmin these classes implement a special method def formfield_for_dbfield(self, db_field, **kwargs) which does all the patching.

For our example we assume that there is new model called Image. It's definition is left out for simplicity. Our News model inlines the new model:

from django.contrib import admin
from modeltranslation.admin import TranslationTabularInline

class ImageInline(TranslationTabularInline):
    model = Image

class NewsAdmin(admin.ModelAdmin):
    list_display = ('title',)
    inlines = [ImageInline,]

admin.site.register(News, NewsAdmin)

Note: In this example only the Image model is registered in translation.py. It's not a requirement that NewsAdmin derives from TranslationAdmin in order to inline a model which is registered for translation.

In this more complex example we assume that the News and Image models are registered in translation.py. The News model has an own custom admin class and the Image model an own generic stacked inline class. It uses the technique described in TranslationAdmin in combination with other admin classes.:

from django.contrib import admin
from modeltranslation.admin import TranslationAdmin, TranslationGenericStackedInline

class TranslatedImageInline(ImageInline, TranslationGenericStackedInline):
    model = Image

class TranslatedNewsAdmin(NewsAdmin, TranslationAdmin):
    def formfield_for_dbfield(self, db_field, **kwargs):
        field = super(TranslatedNewsAdmin, self).formfield_for_dbfield(db_field, **kwargs)
        self.patch_translation_field(db_field, field, **kwargs)
        return field

    inlines = [TranslatedImageInline,]

admin.site.register(News, NewsAdmin)

The update_translation_fields command

In case the modeltranslation app was installed on an existing project and you have specified to translate fields of models which are already synced to the database, you have to update your database schema manually.

Unfortunately the newly added translation fields on the model will be empty then, and your templates will show the translated value of the fields (see Rule 1 below) which will be empty in this case. To correctly initialize the default translation field you can use the update_translation_fieldscommand:

manage.py update_translation_fields

Taken the News example from above this command will copy the value from the news object's title field to the default translation field title_de. It only does so if the default translation field is empty otherwise nothing is copied.

Note: The command will examine your settings.LANGUAGES variable and the first language declared there will be used as the default language.

All translated models (as specified in the project's translation.py will be populated with initial data.

Caveats

Consider the following example (assuming the default lanuage is de):

>>> n = News.objects.create(title="foo")
>>> n.title
'foo'
>>> n.title_de
>>>

Because the original field title was specified in the constructor it is directly passed into the instance's __dict__ and the descriptor which normally updates the associated default translation field (title_de) is not called. Therefor the call to n.title_de returns an empty value.

Now assign the title, which triggers the descriptor and the default translation field is updated:

>>> n.title = 'foo'
>>> n.title_de
'foo'
>>>

Accessing translated fields outside views

Since the modeltranslation mechanism relies on the current language as it is returned by the get_language function care must be taken when accessing translated fields outside a view function.

Within a view function the language is set by Django based on a flexible model described at How Django discovers language preference which is normally used only by Django's static translation system.

When a translated field is accessed in a view function or in a template, it uses the django.utils.translation.get_language function to determine the current language and return the appropriate value.

Outside a view (or a template), i.e. in normal Python code, a call to the get_language function still returns a value, but it might not what you expect. Since no request is involved, Django's machinery for discovering the user's preferred language is not activated. todo: explain more

The unittests in tests.py use the django.utils.translation.trans_realfunctions to activate and deactive a specific language outside a view function.

Related projects

django-multilingual

A library providing support for multilingual content in Django models.

It is not possible to reuse existing models without modifying them.

django-multilingual-model

A much simpler version of the above django-multilingual. It works very similiar to the django-multilingual approach.

transdb

Django's field that stores labels in more than one language in database.

This approach uses a specialized Field class, which means one has to change existing models.

i18ndynamic

This approach is not developed any more.

django-pluggable-model-i18n

This app utilizes a new approach to multilingual models based on the same concept the new admin interface uses. A translation for an existing model can be added by registering a translation class for that model.

This is more or less what modeltranslation does, unfortunately it is far from being finished.

Comment by project member jaap%eig...@gtempaccount.com, Jun 18, 2010

Note that modeltranslation currently only translates text based fields. Integers, Booleans and other non-text db fields are not supported. It's easy to miss but somewhere near the middle it says:

"The modeltranslation app can translate CharField? and TextFieldbased? fields of any model class".

And it does so very well!

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