metaclass for google.appengine.ext.db.Model with i18n properties.

meta.py

from copy import deepcopy

from google.appengine.ext import db

import settings


class I18nModelMetaClass(db.PropertiedClass):
    """
    Metaclass for adding language specific attributes to a
    :class:`~google.appengine.ext.db.Model`

    Make sure that you've got a settings.py with a ``LANGUAGES`` key and
    a DEFAULT_LANGUAGE key::
    
        DEFAULT_LANGUAGE = 'en'
        LANGUAGES = ('en', 'nl', 'de')
    
    usage::
    
        from meta.i18n import I18nModelMetaClass
        from google.appengine.ext import db
        
        class SomeModel(db.Model):
            __metaclass__ = I18nModelMetaClass
            
            # tell I18nModelMetaClass which properties need to internationalized
            i18n = ('title',)
            title = db.StringProperty()
            body = db.TextProperty()
        
        now you will have a title for each language in LANGUAGES; ``title_en``,
        ``title_nl`` and ``title_de``.
    """
    def __new__(cls, name, bases, dct, map_kind=True):
        i18n = dct.pop('i18n')
        
        i18n_attrs = {'_i18n':i18n}
        
        for prop_name, prop in dct.items():
            if prop_name in i18n:
                for language in settings.LANGUAGES:
                    prop_i18n_name = '%s_%s' % (prop_name, language)
                    
                    if language != settings.DEFAULT_LANGUAGE:
                        prop_cpy = deepcopy(prop)
                        # the other languages must not be required, otherwise I
                        # can not make a form that shows only fields for 1 language.
                        prop_cpy.required = False
                        i18n_attrs[prop_i18n_name] = prop_cpy
                    else:
                        i18n_attrs[prop_i18n_name] = prop
                                            
            else:
                i18n_attrs[prop_name] = prop
        
        return db.PropertiedClass(name, bases, i18n_attrs)