Dont work has one retation

Hi! i am use ActiveRow

$clients = $this->database->table('clients');
        foreach($clients as $client){
            dump($client);
            dump($client->ref('clients_enterpneur', 'id_client'));
            echo '<hr>';
        }

have error

No reference found for $clients->ref(clients_enterpneur)

use has many, it work

$clients = $this->database->table('clients');


foreach($clients as $client){
    dump($client);
    foreach($client->related('clients_enterpneur.id_client') as $item){
        dump($item);
    }
    echo '<hr>';
}

DB
clients
CREATE TABLE IF NOT EXISTS clients (
id int(11) NOT NULL,
legal_type varchar(1) NOT NULL
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;

clients_enterpneur
CREATE TABLE IF NOT EXISTS clients_enterpneur (
id_client int(11) NOT NULL,
name varchar(45) DEFAULT NULL,
person_first_name varchar(45) DEFAULT NULL,
person_last_name varchar(45) DEFAULT NULL,
ICO decimal(10,0) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

what is the problem?