Improve private state property checks

When subscribing/unsubscribing, all events for either a student or a course
will be loaded on reconsititution. This also means subscribing events for
a student on other courses, and subscribing events for a course for other
students.

When this has happened before the course and student for the current
subscribe/unsubscribe, certain private state properties are not set yet
(as the required created events did not happen yet for this domain id combination).

This will result in uninitialized errors. Therefore, make sure this cannot
happen by setting defaults.

Author: Jeroen de Graaf

src/Domain/StudentToCourseSubscription/SubscribeStudentToCourse.php

@@ -39,8 +39,8 @@ final class SubscribeStudentToCourse implements EventSourcedDomainContext
     /*
      * Use private properties to guard idempotency and protect invariants.
      */
-    private bool $isStudentSubscribedToCourse;
     private int $courseCapacity;
+    private bool $isStudentSubscribedToCourse;
     private int $totalCountSubscriptionsForCourse;
     private int $totalCountSubscriptionsForStudent;
 
@@ -55,7 +55,7 @@ public function subscribe(): void
         /*
          * Guard for idempotency.
          */
-        if ($this->isStudentSubscribedToCourse) {
+        if ($this->isStudentSubscribedToCourse ?? false) {
             return;
         }
 
@@ -83,7 +83,7 @@ public function subscribe(): void
          */
         $this->apply(new StudentSubscribedToCourseEvent((string) $this->courseId, (string) $this->studentId));
 
-        if ($this->totalCountSubscriptionsForCourse+1 >= $this->courseCapacity) {
+        if ($this->totalCountSubscriptionsForCourse >= $this->courseCapacity) {
             $this->apply(new CourseFullyBookedEvent((string) $this->courseId));
         }
     }
@@ -105,7 +105,6 @@ private function onStudentCreatedEvent(StudentCreatedEvent $event): void
     {
         $this->studentId = new StudentId($event->studentId);
         $this->totalCountSubscriptionsForStudent = 0;
-        $this->isStudentSubscribedToCourse = false;
     }
 
     #[DomainEventSubscriber]
@@ -117,28 +116,38 @@ private function onCourseCapacityChangedEvent(CourseCapacityChangedEvent $event)
     #[DomainEventSubscriber]
     private function onStudentSubscribedToCourseEvent(StudentSubscribedToCourseEvent $event): void
     {
-        if (isset($this->studentId)
-            && $this->studentId->equals(new StudentId($event->studentId))
-            && $this->courseId->equals(new CourseId($event->courseId))
-        ) {
-            ++$this->totalCountSubscriptionsForStudent;
+        $studentId = $this->studentId ?? null;
+        $courseId = $this->courseId ?? null;
+
+        if ($studentId?->equals(new StudentId($event->studentId)) && $courseId?->equals(new CourseId($event->courseId))) {
             $this->isStudentSubscribedToCourse = true;
         }
 
-        ++$this->totalCountSubscriptionsForCourse;
+        if ($courseId?->equals(new CourseId($event->courseId))) {
+            ++$this->totalCountSubscriptionsForCourse;
+        }
+
+        if ($studentId?->equals(new StudentId($event->studentId))) {
+            ++$this->totalCountSubscriptionsForStudent;
+        }
     }
 
     #[DomainEventSubscriber]
     private function onStudentUnsubscribedFromCourseEvent(StudentUnsubscribedFromCourseEvent $event): void
     {
-        if (isset($this->studentId)
-            && $this->studentId->equals(new StudentId($event->studentId))
-            && $this->courseId->equals(new CourseId($event->courseId))
-        ) {
-            --$this->totalCountSubscriptionsForStudent;
+        $studentId = $this->studentId ?? null;
+        $courseId = $this->courseId ?? null;
+
+        if ($studentId?->equals(new StudentId($event->studentId)) && $courseId?->equals(new CourseId($event->courseId))) {
             $this->isStudentSubscribedToCourse = false;
         }
 
-        --$this->totalCountSubscriptionsForCourse;
+        if ($courseId?->equals(new CourseId($event->courseId))) {
+            --$this->totalCountSubscriptionsForCourse;
+        }
+
+        if ($studentId?->equals(new StudentId($event->studentId))) {
+            --$this->totalCountSubscriptionsForStudent;
+        }
     }
 }

src/Domain/StudentToCourseSubscription/UnsubscribeStudentFromCourse.php

@@ -49,7 +49,7 @@ public function unsubscribe(): void
         /*
          * Guard for idempotency.
          */
-        if (!$this->isStudentSubscribedToCourse) {
+        if (!($this->isStudentSubscribedToCourse ?? false)) {
             return;
         }
 
@@ -84,27 +84,26 @@ private function onCourseCreatedEvent(CourseCreatedEvent $event): void
     private function onStudentCreatedEvent(StudentCreatedEvent $event): void
     {
         $this->studentId = new StudentId($event->studentId);
-        $this->isStudentSubscribedToCourse = false;
     }
 
     #[DomainEventSubscriber]
     private function onStudentSubscribedToCourseEvent(StudentSubscribedToCourseEvent $event): void
     {
-        if (isset($this->studentId)
-            && $this->studentId->equals(new StudentId($event->studentId))
-            && $this->courseId->equals(new CourseId($event->courseId))
-        ) {
+        $studentId = $this->studentId ?? null;
+        $courseId = $this->courseId ?? null;
+
+        if ($studentId?->equals(new StudentId($event->studentId)) && $courseId?->equals(new CourseId($event->courseId))) {
             $this->isStudentSubscribedToCourse = true;
         }
     }
 
     #[DomainEventSubscriber]
     private function onStudentUnsubscribedFromCourseEvent(StudentUnsubscribedFromCourseEvent $event): void
     {
-        if (isset($this->studentId)
-            && $this->studentId->equals(new StudentId($event->studentId))
-            && $this->courseId->equals(new CourseId($event->courseId))
-        ) {
+        $studentId = $this->studentId ?? null;
+        $courseId = $this->courseId ?? null;
+
+        if ($studentId?->equals(new StudentId($event->studentId)) && $courseId?->equals(new CourseId($event->courseId))) {
             $this->isStudentSubscribedToCourse = false;
         }
     }