DataMapper ORM v1.8.2

[eluser]Maglok[/eluser]
I have a quick query. You might remember my question about saving advanced relationships. The fix you suggested on the previous page did work, but it does not cover it 100%.

Scenario: Car->wheel->wheelcap. Imagine all advanced relationships.

Code:

//pseudocode
$car = new Car();
$wheel = new Wheel();
$wheelcap = new Wheelcap();

$wheel->save_wheelcap($wheelcap);
$car->save_wheel($wheel);

I then always get a error that is basically 'trying to save but wheel_id in wheelcap object does not have a default value'. Normally datamapper would save the parent and then pass the parent id to the child to use, but it seems it doesn't do that in this scenario.

What am I missing?