Drools: prevent firing for each permutation of a rule - set

Question

Is there a way in drools to check for a set of conditions, rather than a pair of conditions? What I mean by this is let's say I have a fact type called Ticket, and I've inserted two Ticket facts with the same owner field; how do I prevent the following rule from firing twice? (once where A=$t1,B=$t2, and once where B=$t1,A=$t2)
rule "two tickets purchased by same person"
when
$t1 : Ticket($o : owner)
$t2 : Ticket(owner == $o, this != $t1)
then
do something...
end
There are a multitude of ways to do this with non-drools constructs (a member flag the rule flips for example), but is there a way to do this type of check with a native drools construct (and keep the facts in the knowledge base), rather than use a java workaround
Thanks

Answer 1

The standard design pattern is to use a key attribute to force an order on the pair. Tickets might have a serial number:
$t1 : Ticket($o : owner, $sno: serialNumber )
$t2 : Ticket(owner == $o, serialNumber > $sno )
This eliminates the need for the constraint forcing different objects.
But three or more tickets would still create a similar problem. Therefore, you might also keep track of the tickets of an owner:
rule "insert Owner"
when
$t: Ticket( $o: owner )
not TicketSet( owner == $o )
then
insert( new TicketSet( $t ) );
end
rule "more tickets of one Owner"
when
$t: Ticket( $o: owner )
$s: TicketSet( owner == $o, set not contains $t )
then
modify( $s ){ add( $t ) }
// $o buys n-th ticket
end