[support] Computed field, how to acces Foreignkey's data

[Marolijo - Pol maresma]

Hello, from a computed field I wanna to a typical bill-line calculation
doing a multiplication 
price = quantity * fk_product_price
 
How can I do that? I tried wihout luck.
 
$node_field[0]['value'] = $node->field_quantitat[0]['value'] *
$node->field_fk_producte[0]['value']......;
 
 
Thank's!
 
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