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+.. _ref-topics-modeltranslation:
+ Model translation
+.. admonition:: About this document
+    This document provides an introduction to the modeltranslation application.
+.. currentmodule:: modeltranslation.models
+.. moduleauthor:: P. Eschler <>
+The modeltranslation application can be used to translate dynamic content of 
+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.
+The advantage of a registration approach is the ability to add translations to
+models on a per-project basis. You can use the same app in different projects,
+may they use translations or not, and you never have to touch the original 
+model class.
+.. contents::
+- Unlimited number of target languages
+- Add translations without changing existing models
+- Django admin support 
+- ?Supports inherited models
+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 ````. 
+2. Configure your languages in the ````.
+3. Create a ```` in your project directory and register
+   ``TranslationOptions`` for every model you want to translate.
+4. Configure the ``TRANSLATION_REGISTRY`` variable in your ````.
+5. Sync the database using `` syncdb`` (note that this only applies 
+   if the models registered in the ```` 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 ````
+The following variables have to be added to or edited in the project's 
+Make sure that the ``modeltranslation`` app is listed in your 
+``INSTALLED_APPS`` variable::
+        ...
+        'modeltranslation',
+        ....
+    )
+Also make sure that the app can be found on a path contained in your 
+``PYTHONPATH`` environment variable.
+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.
+In order to be able to import the project's ```` 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 ``TextField`` 
+based 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 
+1. Create a ```` 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 ```` 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.ModelTranslation`` 
+and 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 
+          ```` files in the project **and** application directories, 
+          the modeltranslation app looks only for one ```` 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 ```` file in your project directory and 
+add the following::
+    from modeltranslation.translator import translator, TranslationOptions    
+    from 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::
+    $ ./ sqlall news
+    BEGIN;
+    CREATE TABLE `news_news` (
+        `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 ````. 
+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
+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. 
+.. _set_language:
+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_en`` 
+fields 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 
+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',)
+, 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
+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.
+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_fields`` 
+ 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 ```` will be
+populated with initial data. 
+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. 
+.. _How Django discovers language preference:
+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 ```` use the ``django.utils.translation.trans_real``
+functions to activate and deactive a specific language outside a view function.
+Related projects
+    A library providing support for multilingual content in Django models. 
+It is not possible to reuse existing models without modifying them.
+A much simpler version of the above `django-multilingual`.
+It works very similiar to the `django-multilingual` approach. 
+    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. 
+This approach is not developed any more.
+    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.
+.. _django-multilingual:
+.. _django-multilingual-model:
+.. _django-transdb:
+.. _i18ndynamic:
+.. _django-pluggable-model-i18n: