There are several ways of doing that, and some where even mentioned by the other users. One easy way is to simply use "accumulate":

rule "update amount"
    no-loop
when
    $f : Fact( name == "amount" )
    Number( $total : doubleValue ) from accumulate(
              Charge( $amt : amount ),
              sum( $amt ) )
then
    modify( $f ) { setAmount( $total ) }
end

    []s
    Edson

2008/11/4 Dan Seaver <dan.seaver@ge.com>

I'm trying to find a good technique for updating specific facts in working
memory. What I'm currently doing is something like this:

Rule "Update Amount"
  when
     amountFact : Fact(name == "Amount")
     charge : Charge()
  then
     Double amount = amountFact.getAmount();
     Double chargeAmount = charge.getAmount();
     amountFact.setAmount(amount + chargeAmount);
     update(amountFact);
end

The update statement causes an infinite loop.
I tried using no-loop, which works if there is 1 charge, but not if there
are more than one.

Any help with solutions or strategies would be much appreciated.
--
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 Edson Tirelli
 JBoss Drools Core Development
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