Howtos
...Retrieve all objects not yet associated (in an has_many association with a join table)
The following exemple will retrieve all users which are not member of any group.
$user = new User();
$res = $user->execute('
SELECT Users.*
FROM Users
LEFT OUTER JOIN Groups_Users
ON Groups_Users.user_id = Users.id
WHERE Groups_Users.id IS NULL');
$users = array();
while($row = mysql_fetch_assoc($res)){
$user = new User();
$user->fromArray($row);
$users[] = $user;
}
print_r($users);