Optaplanner Curriculum example, Explanation for curriculumCourseScoreRules.drl

Question

I'm currently reading the course curriculum example of optaplanner and I can't seem to understand this:

rule "conflictingLecturesSameCourseInSamePeriod"
    when
        // line 1
        Lecture($leftId : id, $leftCourse : course, $period : period, period != null)
        // line 2
        Lecture(course == $leftCourse, period == $period, id > $leftId)
    then
        scoreHolder.addHardConstraintMatch(kcontext, -1);
end

Questions are:
- What's the difference between the Lecture() in line 1 and the Lecture in line 2?
- I understand the variable assignements are happening in Line 1 but in Line 2, what's the difference between course and $leftCourse, period and $period and id and $leftId

So far, I can't seem to find any explanation in the documentation

Answer 1

A binding such as $leftCourse : course establishes $leftCourse as a variable that refers to the field course of a Lecture object.

The lecture in line 1 is identified by its id; the one in line 2 has an id that is greater. Assuming that id values are primary keys, this combination would basically match all possible unordered pairs of Lectures, but ...

... the constraint course == $leftCourse restricts the pairings to those of identical courses and ...

... the constraint period == $period furthermore restricts it to equal (non-null) periods.

In other words, the planning rules out an assignment for two different Lectures for the same course in the same period.