public function initAction(Model $m, $fields = null)
    {
        $keys = $fields ? array_flip($fields) : null;

        $data = array_map(function ($r) use ($m, $keys) {
            return $this->typecastLoadRow($m, $keys ? array_intersect_key($r, $keys) : $r);
        }, $this->data[$m->table]);

        return new \atk4\data\Action\Iterator($data);
    }