tickets: 16508

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id created changetime last_pulled_from_trac stage status component type severity version resolution summary description owner reporter keywords easy has_patch needs_better_patch needs_tests needs_docs ui_ux
16508 2011-07-24 11:36:29 2019-03-08 06:15:51 2022-03-06 04:03:24.785060 Accepted assigned Database layer (models, ORM) New feature Normal dev   Provide real support for virtual fields I have created a virtual field based on `GenericForeignKey`, but I found out that it can not be properly used for model initiation. Altered example from my test: {{{ class Managing(models.Model): shortcut = models.CharField(max_length=20) name_cs = TranslationProxyField('name', 'cs', False) class ManagingTranslation(models.Model): name = models.CharField(max_length=20) class TranslationProxyField(object): """ Descriptor that is a virtual field that provides access to translations. """ def __init__(self, field_name, language_code=None, fallback=False): self._field_name = field_name self._language_code = language_code self._fallback = fallback names = [field_name] if language_code is not None: names.append(language_code.replace('-', '_')) if fallback: names.append(FALLBACK_FIELD_SUFFIX) self.name = '_'.join(names) def contribute_to_class(self, cls, name): if self.name != name: raise ValueError('Field proxy %s is added to class under bad attribute name.' % self) self.model = cls cls._meta.add_virtual_field(self) # Connect myself as the descriptor for this field setattr(cls, name, self) # This results in Type error from Model.__init__, because of extra kwarg Managing(shortcut='name', name_cs='jméno') }}} I tried to use `pre_init` signal, but I found out that I will not get `instance` argument, so I can not use that. This way is truly usable only for virtual fields which are like `GenericForeignKey` built on real model fields of object. When it gets more complicated, it can not be used. Confusing on this path is that `instance` is in `providing_args` of `pre_init` signal. Currently I solve this by quite a hack which is deriving of `TranslationProxyField` from `property` instead of `object`. This works because of setting properties is allowed in `Model.__init__`. In my opinion, a stable solution would be either providing an `instance` argument in `pre_init` signal or enable setting of virtual field in model initiation. Second option would also enable removal of `pre_init` connection from `GenericForeignKey`. Exact examples can be found at github: https://github.com/vzima/django-multilingual-ds9 auvipy vzima   0 0 0 0 0 0