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ModelForms

ModelForms for multilanguage models are defined and handled as typical ModelForms. Please note, however, that they shouldn't be defined next to models (see :ref:`a note <register-precautions>`).

Editing multilanguage models with all translation fields in the admin backend is quite sensible. However, presenting all model fields to the user on the frontend may be not the right way. Here comes the TranslationModelForm which strip out all translation fields:

from news.models import News
from modeltranslation.forms import TranslationModelForm

class MyForm(TranslationModelForm):
    class Meta:
        model = News

Such a form will contain only original fields (title, text - see :ref:`example <registration>`). Of course, upon saving, provided values would be set on proper attributes, depending on the user current language.

Formfields and nullability

Note

Please remember that all translation fields added to model definition are nullable (null=True), regardless of the original field nullability.

In most cases formfields for translation fields behave as expected. However, there is one annoying problem with models.CharField - probably the most commonly translated field type.

The problem is that default formfield for CharField stores empty values as empty strings (''), even if the field is nullable (see django ticket #9590).

Thus formfields for translation fields are patched by modeltranslation. The following rules apply:

  • If the original field is not nullable, an empty value is saved as '';
  • If the original field is nullable, an empty value is saved as None.

To deal with complex cases, these rules can be overridden per model or even per field using TranslationOptions:

class NewsTranslationOptions(TranslationOptions):
    fields = ('title', 'text',)
    empty_values = None

class ProjectTranslationOptions(TranslationOptions):
    fields = ('name', 'slug', 'description',)
    empty_values = {'name': '', 'slug': None}

If a field is not mentioned while using dict syntax, the :ref:`default rules <formfield_rules>` apply.

This configuration is especially useful for fields with unique constraints:

class Category(models.Model):
    name = models.CharField(max_length=40)
    slug = models.SlugField(max_length=30, unique=True)

Because the slug field is not nullable, its translation fields would store empty values as '' and that would result in an error when two or more Categories are saved with slug_en empty - unique constraints wouldn't be satisfied. Instead, None should be stored, as several None values in the database don't violate uniqueness:

class CategoryTranslationOptions(TranslationOptions):
    fields = ('name', 'slug')
    empty_values = {'slug': None}

None-checkbox widget

Maybe there is a situation where you want to store both - empty strings and None values - in a field. For such a scenario there is a third configuration value: 'both':

class NewsTranslationOptions(TranslationOptions):
    fields = ('title', 'text',)
    empty_values = {'title': None, 'text': 'both'}

It results in a special widget with a None-checkbox to null a field. It's not recommended in frontend as users may be confused what this None is. The only useful place for this widget might be the admin backend; see :ref:`admin-formfield`.

To sum it up, the valid values for empty_values are: None, '' and 'both'.