Q:

Nested relationships for normalized db structure in FuelPHP ORM

First up, sorry for the mega-posting of code but...
I have the following DB structure (not my choice):
+---------+    +--------------+    +--------+
|  users  |    | users_groups |    | groups |
+---------+    +--------------+    +--------+
|   id    |    |   user_id    |    |   id   |
|  name   |    |   group_id   |    |  name  |
+---------+    +--------------+    +--------+

I've created a relationship with FuelPHP's ORM:
class Model_User extends Orm\Model
{
    protected static $_properties = array(
        'id',
        'name',
    );

    protected static $_has_many = array('usergroup');
}

class Model_Usergroup extends Orm\Model
{
    protected static $_table_name = 'users_groups';

    protected static $_properties = array(
        'user_id',
        'group_id',
    );

    protected static $_belongs_to = array('user', 'group');
}

class Model_Group extends Orm\Model
{
    protected static $_properties = array(
        'id',
        'name',
    );

    protected static $_has_many = array('usergroup');
}

And I'm now trying to get a query to display all the data from the 3 tables using the following:
$data['users'] = Model_User::find()
    ->related('usergroup')
    ->related('usergroup.group')
    ->get();

It's returning this:
Array
(
    [1] => Model_User Object
        (
            [_is_new:Orm\Model:private] => 
            [_frozen:Orm\Model:private] => 
            [_data:Orm\Model:private] => Array
                (
                    [id] => 1
                    [name] => somename
                )

            [_original:Orm\Model:private] => Array
                (
                    [id] => 1
                    [name] => somename
                )

            [_data_relations:Orm\Model:private] => Array
                (
                    [usergroup] => Array
                        (
                        )

                )

            [_original_relations:Orm\Model:private] => Array
                (
                )

            [_view:Orm\Model:private] => 
            [_iterable:protected] => Array
                (
                )
        )
)

So it's starting to find the users_groups but stops? Am I even getting at this in the right way? Of course I could get this with a regular query with joins, but it would be ace to actually know how to do this with the ORM.

A:

Gaaaak! It was just missing a PK on the users_groups table... 
class Model_Usergroup extends Orm\Model
{
    protected static $_table_name = 'users_groups';
    protected static $_primary_key = array('pk_id');

    protected static $_properties = array(
        'pk_id',
        'user_id',
        'group_id',
    );

    protected static $_belongs_to = array('user', 'group');
}

