[rules-users] Why Using "from" Always Return A New Fact?

[Edson Tirelli]

   As of Drools 5.1.1, drools looks at the expression in "from" as a
black box. Every time it is executed, drools creates a new fact handle
to wrap the result(s). Since the lock and no-loop features are based
on the fact handles, the sometimes undesired interaction occurs. We
may be able to improve this in drools 6, when we will have the engine
owning 100% of the language syntax, that will enable us to run better
expression analisys than we can do today.

   Edson

Em quarta-feira, 19 de janeiro de 2011,
hyjshanghai<hyjshanghai at gmail.com> escreveu:
>
> You explanation is very reasonable:
> the engine assumes anything within $p may be changed by modify($p), although
> $p.address is not changed actually.
>
> However, I tried these rules myself and both rules were fired. Why?
> According to the document, only one of the rules should fire; the other's
> activation should be cancelled because the engine assumes $p has changed,
> which may mismatch the other rule, right?.
>
> The following are the output and my rules. The only difference between my
> rules and the document's is that my rules don't belong to any
> ruleflow-group.
>
> ==== Program Output of Inserting a "new Person()" and Firing All Rules.
> BEGIN====
> Rule 'Apply discount' fired. Person: Person{name[Tom],
> address[Address{state[NC], city[Raleigh]}], region[null], discount[0.9]}
> Rule 'Assign to sales region 1' fired. Person: Person{name[Tom],
> address[Address{state[NC], city[Raleigh]}], region[sales region 1],
> discount[0.9]}
> ==== Program Output of Inserting a "new Person()" and Firing All Rules.
> END====
>
> ==== The Rules I Tested. BEGIN ====
> package hello.rules
> import java.util.*
> import java.math.*
> import hello.model.Address;
> import hello.model.Person;
>
> rule "Assign to sales region 1"
> lock-on-active true
> when
>     $p : Person()
>     $a : Address( state=="NC") from $p.address
> then
>     modify ($p) { setRegion("sales region 1") }
>     System.out.println("Rule 'Assign to sales region 1' fired. Person:
> "+$p);
> end
>
> rule "Apply discount"
> lock-on-active true
> when
>     $p : Person()
>     $a : Address( city=="Raleigh") from $p.address
> then
>     modify ($p) { setDiscount((float)0.9) }
>     System.out.println("Rule 'Apply discount' fired. Person: "+$p);
> end
> ==== The Rules I Tested. END ====
>
> ==== The Person and Address. BEGIN ====
> public class Person
> {
>     private String name;
>     private Address address;
>     private String region;
>     private float discount;
>     public String getRegion() {
>         return region;
>     }
>     public void setRegion(String region) {
>         this.region = region;
>     }
>     public float getDiscount() {
>         return discount;
>     }
>     public void setDiscount(float discount) {
>         this.discount = discount;
>     }
>
>     public String getName() {
>         return name;
>     }
>     public void setName(String name) {
>         this.name = name;
>     }
>     public Address getAddress() {
>         return address;
>     }
>     public void setAddress(Address address) {
>         this.address = address;
>     }
>     @Override
>     public String toString()
>     {
>         return String.format("Person{name[%s], address[%s], region[%s],
> discount[%s]}",
>                 name, address.toString(), region, discount);
>     }
> }
>
> public class Address
> {
>     private String state;
>     private String city;
>     public String getState() {
>         return state;
>     }
>     public void setState(String state) {
>         this.state = state;
>     }
>     public String getCity() {
>         return city;
>     }
>     public void setCity(String city) {
>         this.city = city;
>     }
>
>     @Override
>     public String toString()
>     {
>         return String.format("Address{state[%s], city[%s]}", state, city);
>     }
> }
> ==== The Person and Address END ====
> --
> View this message in context: http://drools-java-rules-engine.46999.n3.nabble.com/Why-Using-from-Always-Return-A-New-Fact-tp2286393p2292029.html
> Sent from the Drools - User mailing list archive at Nabble.com.
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-- 
  Edson Tirelli
  JBoss Drools Core Development
  JBoss by Red Hat @ www.jboss.com