In-Table Foreign Keys in DataMapper ORM

[eluser]tarciozemel[/eluser]
OK, so I not understood very well... Let's some example. I want a 1:N relationship according this:

user_model.php

Code:

var $has_one = array(
    'user_group' => array(
        'class' => 'user_group_model'
    )
);

user_group_model.php

Code:

// In the "group" table, I have only the "id", "name" and "desc" colums
var $has_many = array(
    'user' => array(
        'class' => 'user_model'
        'join_self_as' => 'group' // In the "user" table, exists a "group_id" colum
    )
);

When I try do this:

Code:

$user = new User_model();
$user->where('login', 'foobar')->get();
$user->user_group->get();
echo $user->user_group->desc;

I give this error:

DataMapper Error: 'User_model' is not a valid parent relationship for User_group_model. Are your relationships configured correctly?

So, where's my mistake? I have to specify a "join_self_as" in the user_model even the other table not having a relationship field? If yes, how?

Regards!