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

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!