hasOne not working

I have these 2 tables:

CREATE TABLE IF NOT EXISTS `persons` (
    `person_id` int(11) NOT NULL AUTO_INCREMENT,
    `str_name` varchar(50) NOT NULL,
    `str_last_name` varchar(50) NOT NULL,
    `dt_birthday` date NOT NULL,
    `country_id` int(11) DEFAULT NULL,
    PRIMARY KEY (`person_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=2 ;

CREATE TABLE IF NOT EXISTS `users` (
    `user_id` int(11) NOT NULL AUTO_INCREMENT,
    `str_usr` varchar(35) NOT NULL,
    `str_email` varchar(50) NOT NULL,
    `str_pwd` varchar(40) NOT NULL,
    `person_id` int(11) NOT NULL,
    PRIMARY KEY (`user_id`),
    UNIQUE KEY `indx_person_id` (`person_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

ALTER TABLE `users`
    ADD CONSTRAINT `users_ibfk_1` FOREIGN KEY (`person_id`) REFERENCES `persons` (`person_id`) ON DELETE CASCADE ON UPDATE NO ACTION;

Now, I have these 2 models:

class Person extends \Phalcon\Mvc\Model {

    public function getSource() {
        return "persons";
    }

    public function initialize() {

    }

    public function columnMap() {
        return array(
            'person_id' => 'id',
            'str_name' => 'name',
            'str_last_name' => 'lastName',
            'dt_birthday' => 'dBirhtday',
            'country_id' => 'country'
        );
    }
}

And

class User extends \Phalcon\Mvc\Model {

    public function getSource() {
        return "users";
    }

    public function initialize() {
        $this->hasOne("person", "Person", "id");
    }

    /**
     * Mapeamento das colunas do db para aplicação
     * @return array
     */
    public function columnMap() {
        return array(
            'user_id' => 'id',
            'str_usr' => 'usrName',
            'str_email' => 'email',
            'str_pwd' => 'password',
            'person_id' => 'person'
        );
    }

    public $person; //field to load the person
}

I omitted the getters/setters and fields due the space.

When I try to load the person from a user it is null. Why???