def _cache_keys(self):
    """Return the cache key for self plus all related foreign keys."""
    fks = dict((f, getattr(self, f.attname)) for f in self._meta.fields
                if isinstance(f, models.ForeignKey))

    keys = [fk.rel.to._cache_key(val, self._state.db) for fk, val in fks.items()
            if val is not None and hasattr(fk.rel.to, '_cache_key')]
    return (self.cache_key,) + tuple(keys)