Thank you Mike for the correction, yes I meant the DRL.
Hi,
I assume you mean Drools DRL (DSL is something different and none of your examples suggest use of this so far).
How about checking for the absence of the Double instead of it being null:-
when
$product : Product( )
not Double( ) from bookingService.getQty( $product )
I've not tried it personally.
With kind regards,
Mike