Notifications: Fixed error on comment notification

Fixes an error where a used relation (entity) on the comment was
resulting in null due to eager loading the notification when
deserializing from the queue, where Laravel was then mis-matching the
names when performing the eager loading.

For #5918

app/Activity/Models/Comment.php

@@ -41,7 +41,19 @@ class Comment extends Model implements Loggable, OwnableInterface
      */
     public function entity(): MorphTo
     {
-        return $this->morphTo('commentable');
+        // We specifically define null here to avoid the different name (commentable)
+        // being used by Laravel eager loading instead of the method name, which it was doing
+        // in some scenarios like when deserialized when going through the queue system.
+        // So we instead specify the type and id column names to use.
+        // Related to:
+        // https://github.com/laravel/framework/pull/24815
+        // https://github.com/laravel/framework/issues/27342
+        // https://github.com/laravel/framework/issues/47953
+        // (and probably more)
+
+        // Ultimately, we could just align the method name to 'commentable' but that would be a potential
+        // breaking change and not really worthwhile in a patch due to the risk of creating extra problems.
+        return $this->morphTo(null, 'commentable_type', 'commentable_id');
     }
 
     /**

app/Activity/Notifications/Handlers/BaseNotificationHandler.php

@@ -20,6 +20,7 @@ abstract class BaseNotificationHandler implements NotificationHandler
     {
         $users = User::query()->whereIn('id', array_unique($userIds))->get();
 
+        /** @var User $user */
         foreach ($users as $user) {
             // Prevent sending to the user that initiated the activity
             if ($user->id === $initiator->id) {