OptaPlanner, how can I get the count of filtered data?

Question

I'm trying to make a course assignment schedule with optaplanner.

How can I get the count of same lessons with the same class on the same day? I am using constraint management

                for example
  -----------------------------------------------

  0.P     Math              Math           Math --> 3  ---Pen.ofHard(5)

  1.P     Math              Math           Another --> 2 -- No Pen

  2.P     Another           Another        Another --> 3  -- Pen.ofHard(5)

(I wanted to use groupBy but I couldn't)

i want to do :

private Constraint checkMaxLesson(ConstraintFactory constraintFactory)    {
    return constraintFactory.fromUniquePair(Lesson.class,
            Joiners.equal(Lesson::getStudentGroup),
            Joiners.equal(Lesson::getSubject),
            Joiners.equal(t -> t.getTimeSlot().getDayOfWeek()))
            **??.filter( count() > 2)**
            .penalize("Max 2 lesson some day",HardSoftScore.ofHard(5));
}
  
          

Answer 1

I am not quite sure why you weren't able to use groupBy(), you should add the explanation to your question. That said, what you're trying to do should be doable like so:

private Constraint checkMaxLesson(ConstraintFactory constraintFactory) {
    return constraintFactory.from(Lesson.class)
        .groupBy(Lesson::getStudentGroup, 
            Lesson::getSubject,
            lesson -> lesson.getTimeSlot().getDayOfWeek(),
            ConstraintCollectors.count())
        .filter((group, subject, dayOfWeek, lessonCount) -> lessonCount > 2)
        .penalize("Max 2 lessons on any given day",
            HardSoftScore.ofHard(5),
            (group, subject, dayOfWeek, lessonCount) -> lessonCount - 2));
}