How to write .drl for list type object

[Jainath Gupta]

Hi,

    I want to write .drl for List of object. I didn't get any ideas. Below code is autogenerated by drools project, but I made little change. My change is inserting object is List type.

public class DroolsTest {

.java

    public static final void main(String[] args) {

        try {

            // load up the knowledge base

       KieServices ks = KieServices.Factory.get();

       KieContainer kContainer = ks.getKieClasspathContainer();

        KieSession kSession = kContainer.newKieSession("ksession-rules");

            // go !

            Message message = new Message();

            message.setMessage("Hello World");

            message.setStatus(Message.HELLO);

            

            Message message1 = new Message();

            message1.setMessage("Welcome");

            message1.setStatus(Message.HELLO);

            

            List<Message> messageList = new ArrayList<Message>();

            messageList.add(message);

            messageList.add(message1);

            

            kSession.insert(messageList);

            kSession.fireAllRules();

        } catch (Throwable t) {

            t.printStackTrace();

        }

    }

    public static class Message {

        public static final int HELLO = 0;

        public static final int GOODBYE = 1;

        private String message;

        private int status;

        public String getMessage() {

            return this.message;

        }

        public void setMessage(String message) {

            this.message = message;

        }

        public int getStatus() {

            return this.status;

        }

        public void setStatus(int status) {

            this.status = status;

        }

    }

}

.drl:- auto generated and this one is for Message object not for List<Message> object.

rule "Hello World"

    when

        m : Message( status == Message.HELLO, myMessage : message )

    then

        System.out.println( myMessage );

        m.setMessage( "Goodbye cruel world" );

        m.setStatus( Message.GOODBYE );

        update( m );

end

Please Help.

[Jainath Gupta]

Hi Team.

  Please answer  this question.

[José Filipe Neis]

Which rule are you trying to write? Check if any msg in your list is of HELLO type? All of them? None of them?

[Jainath Gupta]

Hi Jose,

   I am inserting List of Message kSession.insert(messageList); So above(m : Message( status == Message.HELLO, myMessage : message )) .drl is wrong. Can you Print System.out.println( myMessage ); this message.

Thanks

[José Filipe Neis]

Jainath,

Writing the rules will follow the same kind of pattern you used in your example, but remember that now you have a List. What does this influence? That Drools will match your objects if they are equals (and list.equals(list) is not a trivial thing to match).

If I would give you any advice right now is: avoid using a List as a session object, but I really don't know your business requirements as you didn't talk about them at all so far.

Anyway, to write a rule that matches a list property inside another object, you can use contains/memberof or a condition like the one exposed on this SO question:

rule "some rule name"
when
$a: A($bset : bset)
$b: B(x == "hello") from $bset 
then
//you will have one activation for each of the B objects matching //the second pattern end

Filipe

[Jainath Gupta]

If I would give you any advice right now is: avoid using a List as a session object, 

Hi Jose,

In business requirements it's possible that we can insert list as a session object. Which I did in my java class(Is it wrong ? hope so no). So It was drools team responsibility that they should provide provision for writing rules on  List as a session object.

[José Filipe Neis]

Drools accept any kind of object as a fact, including a list. It's up to you to define how will this list be matched? How equality works around your list? If all elements are equal?

I tend to think that it would be better to insert the individual elements and match them in an aggregated way, but I don't know which requirements you're dealing with. Again: if you tell what are your requirements, maybe there is a better approach them matching a list that people can suggest you.

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