Need to add multiple items in list - decision table - Drools

Question

I need to create a new multiple instance of objects for the Pojo class in drools decision table. I have implemented using two facts Student fact and subject fact class. I need to fire all the rules in the decision table and I need to add all the values into array-list of the objects. But I'm getting only last rule values of decision table. It seems like decision table values are getting overridden.

Fact 1

Class StudentFact{

 private int id;
 private String name;
 private List<SubejctFact> subjectList;

 public void setId(int id){
    this.id = id;
 }

 public int getId(){
    return id;
 } 

 public void setName(String name){
    this.Name = name;
 }    

 public String getName(){
    return name;
 }

     public void setSubjectList(List<Subject> subjectList) {
        this.subjectList = subjectList;
    }



    public int getSubjectList() {
        return subjectList;
    }




}

Fact 2

Class SubjectFact{
 private int subId;
 private String subjectName;

 public void setSubId(int subId){
     this.subId= subId;
 }

 public int getSubId(){
   return subId;
 }

 public void setSubjectName(String subjectName){
      this.subjectName = subjectName;
 }

 public int getSubejctName(){
      return subjectName;
 }

}

Current Response

{
  "id":123,
  "name": "xyz",
  "subjectList": [
     { 
        "id": 6,
        "name":"Hindi"
     },
     {
        "id": 6,
        "name":"Hindi"
     }
}

Expected Response

 {
      "id":123,
      "name": "xyz",
      "subjectList": [
         { 
            "id": 5,
            "name":"English"
         },
         {
            "id": 6,
            "name":"Hindi"
         }
    }

My Decision Table looks like

Any one pls advise to achieve the expected response?

Answer 1

Each row in a table becomes a rule, each action column becomes a row in then block.
For each rule you need a statement to create Subject, statements to populate it and statement to add it to matching student.
Values in 'CREATE' and 'COLLECT' are needed, otherwise action will be skipped.
; is required in a cell without 'target object' and it is not required when you provide '$subject' and '$student' objects. Don't ask me why. Just analyzed generated drl.
You may want to hide two 'technical columns'.

This will generate two rules like below

package draft;
//generated from Decision Table
import draft.Student;
import draft.Subject;
// rule values at A9, header at A4
rule "Rule 1"
    when
        $student:Student(id == "123")
    then
        Subject $subject = new Subject();
        $subject.setSubId(5);
        $subject.setSubjectName('English');
        $student.addSubject($subject);
end

// rule values at A10, header at A4
rule "Rule 2"
    when
        $student:Student(id == "123")
    then
        Subject $subject = new Subject();
        $subject.setSubId(6);
        $subject.setSubjectName('Hindi');
        $student.addSubject($subject);
end

PS: I was struggling with " being automatically replaced by Calc editor to `` which was not valid symbol for drools parser, so I used single quotes, which appeared to be special symbol at the start of a cell in the editor and skipped. So actual cell value which finally worked for me was ''English'.

Here are my models

public class Student {
    private int id;
    private String name;
    private List<Subject> subjectList = new ArrayList<>();
    
    public Student(int id, String name) {
        this.id = id;
        this.name = name;
    }
    
    public void setId(int id) {
        this.id = id;
    }
    
    public int getId() {
        return id;
    }
    
    public void setName(String name) {
        this.name = name;
    }
    
    public String getName() {
        return name;
    }
    
    public void addSubject(Subject subject) {
        subjectList.add(subject);
    }
    
    public void setSubjectList(List<Subject> subjectList) {
        this.subjectList = subjectList;
    }
    
    public List<Subject> getSubjectList() {
        return subjectList;
    }
}

public class Subject {
    private int subId;
    private String subjectName;
    
    public void setSubId(int subId) {
        this.subId = subId;
    }
    
    public int getSubId() {
        return subId;
    }
    
    public void setSubjectName(String subjectName) {
        this.subjectName = subjectName;
    }
    
    public String getSubejctName() {
        return subjectName;
    }
}

test

@DroolsSession(resources = "draft/ApplicableSubjects.xls",
        builderProperties = "drools.dump.dir = target/dump")
public class PlaygroundTest {
    
    @Rule
    public DroolsAssert drools = new DroolsAssert();
    
    @Test
    public void testIt() {
        drools.insertAndFire(new Student(123, "Student 123"));
        drools.printFacts();
    }
}

test output

00:00:00 --> inserted: Student[id=123,name=Student 123,subjectList=[]]
00:00:00 --> fireAllRules
00:00:00 <-- 'Rule 1' has been activated by the tuple [Student]
00:00:00 <-- 'Rule 2' has been activated by the tuple [Student]
00:00:00 Facts (1):
Student[id=123,name=Student 123,subjectList=[draft.Subject@1ded7b14, draft.Subject@29be7749]]