Get object fields used for calculation of django-tutelary object
    paths.

def get_path_fields(cls, base=[]):
    """Get object fields used for calculation of django-tutelary object
    paths.

    """
    pfs = []
    for pf in cls.TutelaryMeta.path_fields:
        if pf == 'pk':
            pfs.append(base + ['pk'])
        else:
            f = cls._meta.get_field(pf)
            if isinstance(f, models.ForeignKey):
                pfs += get_path_fields(f.target_field.model, base=base + [pf])
            else:
                pfs.append(base + [f.name])
    return pfs