[eluser]WanWizard[/eluser]
In your example your source object is empty, so there's nothing to relate to.
If you want the related objects, you should not use the where_related(), because that generates a where clause on the related object (and that needs a valid source (object).
You need
Code:
// get the post record
$post=new Post( $variable_holding_id );
// get the related records
$post->source->get();
$post->courtesy->get();