It happens that I have to instantiate my referenced model before getting the model relation data. Do you need to do that?
$customer = new Customers();
$systemUser = SystemUsers::findFirst();
echo "<pre>"; var_dump($systemUser->customers); echo "</pre>";
//works, show correct database row
//$customer = new Customers();
$systemUser = SystemUsers::findFirst();
echo "<pre>"; var_dump($systemUser->customers); echo "</pre>";
//doesn't work, internal error
update (I might find the cause of the problem)
public function initialize(){
//$this->hasOne('id', 'customers', 'id__system_users'); // I used lowercase 'c' for 'customers', and I will have to instantiate the Customers model
$this->hasOne('id', 'Customers', 'id__system_users'); // it works just like the example from the doc, I get the referenced row without instantiation
}