id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
16508	Provide real support for virtual fields	Vlastimil Zíma		"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"	New feature	new	Database layer (models, ORM)	dev	Normal			pirosb3 michal.petrucha@…	Accepted	0	0	0	0	0	0