In reality it doesn't return any fact, it returns each of the values currently in the list held by the bound fact ($p). Each time the rule is re-evaluated (due to $p changing) it re-iterates over the list again (as it doesn't know if the list has changed or not). Drools effectively doesn't know anything about the $a's returned from the list nor does it keep track of them. Hence when re-evaluating the rule it can't know if it is the same set of values that matches last time or a different set of values.
Thomas > -----Original Message----- > From: rules-users-boun...@lists.jboss.org [mailto:rules-users- > boun...@lists.jboss.org] On Behalf Of hyjshanghai > Sent: 19 January 2011 10:21 > To: rules-users@lists.jboss.org > Subject: [rules-users] Why Using "from" Always Return A New Fact? > > > I am trying to understand why care should be taken to use "from" and > "lock-on-active" together, as is described in "Section 4.8.3.8. Conditional > Element from" of > http://downloads.jboss.com/drools/docs/5.1.1.34858.FINAL/drools- > expert/html/ch04.html > the Drools Reference Document . > > It says "the use of from returns a new fact for all intents and purposes > each time it is evaluated." Why is it true? > > Below is the full related-text copied from the Drools Reference Document for > your reference: > === BEGIN === > You must take caution, however, when using from, especially in conjunction > with the lock-on-active rule attribute as it may produce unexpected results. > Consider the example provided earlier, but now slightly modified as follows: > > rule "Assign people in North Carolina (NC) to sales region 1" > ruleflow-group "test" > lock-on-active true > when > $p : Person( ) > $a : Address( state == "NC") from $p.address > then > modify ($p) {} #Assign person to sales region 1 in a modify block > end > > rule "Apply a discount to people in the city of Raleigh" > ruleflow-group "test" > lock-on-active true > when > $p : Person( ) > $a : Address( city == "Raleigh") from $p.address > then > modify ($p) {} #Apply discount to person in a modify block > end > > In the above example, persons in Raleigh, NC should be assigned to sales > region 1 and receive a discount; i.e., you would expect both rules to > activate and fire. Instead you will find that only the second rule fires. > > If you were to turn on the audit log, you would also see that when the > second rule fires, it deactivates the first rule. Since the rule attribute > lock-on-active prevents a rule from creating new activations when a set of > facts change, the first rule fails to reactivate. Though the set of facts > have not changed, the use of from returns a new fact for all intents and > purposes each time it is evaluated. > === END === > > -- > View this message in context: http://drools-java-rules- > engine.46999.n3.nabble.com/Why-Using-from-Always-Return-A-New-Fact- > tp2286393p2286393.html > Sent from the Drools - User mailing list archive at Nabble.com. > _______________________________________________ > rules-users mailing list > rules-users@lists.jboss.org > https://lists.jboss.org/mailman/listinfo/rules-users ************************************************************************************** This message is confidential and intended only for the addressee. If you have received this message in error, please immediately notify the postmas...@nds.com and delete it from your system as well as any copies. The content of e-mails as well as traffic data may be monitored by NDS for employment and security purposes. To protect the environment please do not print this e-mail unless necessary. NDS Limited. Registered Office: One London Road, Staines, Middlesex, TW18 4EX, United Kingdom. A company registered in England and Wales. Registered no. 3080780. VAT no. GB 603 8808 40-00 ************************************************************************************** _______________________________________________ rules-users mailing list rules-users@lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users
