def validate(self, value, model_instance):
        if self.rel.parent_link:
            return
        super(ForeignKey, self).validate(value, model_instance)
        if value is None:
            return

        using = router.db_for_read(model_instance.__class__, instance=model_instance)
        qs = self.rel.to._default_manager.using(using).filter(
                **{self.rel.field_name: value}
             )
        qs = qs.complex_filter(self.rel.limit_choices_to)
        if not qs.exists():
            raise exceptions.ValidationError(self.error_messages['invalid'] + {
                'model': self.rel.to._meta.verbose_name, 'pk': value})
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/db/models/fields/related.py/ForeignKey.validate/operator `%` -> `+`