Allow content_type or object_id of GenericForeignKey to be field on related model

[Chris Morbitzer]

I would like to specify a related model field as the values of content_type or object_id of GenericForeignKey. For example:

class Widget(models.Model):
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)

class Gadget(models.Model):
    widget = models.ForeignKey(Widget, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey('widget__content_type', 'object_id')

If I tried to use this now, I would get the error:

Traceback (most recent call last):
  File "django/db/models/options.py", line 617, in get_field
    return self.fields_map[field_name]
KeyError: 'widget__content_type'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "django/contrib/contenttypes/fields.py", line 224, in __get__
    f = self.model._meta.get_field(self.ct_field)
  File "django/db/models/options.py", line 619, in get_field
    raise FieldDoesNotExist('%s has no field named %r' % (self.object_name, field_name))
django.core.exceptions.FieldDoesNotExist: Gadget has no field named 'widget__content_type'

One use case for this is a generic custom attribute system, where attribute fields can be created for any other model. The "fields" model has the content_type field. The "values" model has the object ID and value fields. The developer would like to query the object that the "values" record references: value.content_object

[Tim Graham]

The usual advise is to avoid GenericForeignKey in the first place. I doubt there would be consensus to add that complexity, if it's even feasible. If you can offer patch, please raise the idea on the DevelopersMailingList to see what other think.