call static instead of self in Model

src/PersistentData/Model/Model.php

@@ -41,7 +41,7 @@ abstract class Model
     {
         $array = [];
 
-        foreach (self::getFields() as $key) {
+        foreach (static::getFields() as $key) {
             $method = 'get' . str_replace('_', '', ucwords($key, '_'));
 
             if (method_exists($this, $method)) {

src/PersistentData/Model/ModelWithSlug.php

@@ -2,7 +2,7 @@
 
 use Cocur\Slugify\Slugify;
 
-class ModelWithSlug extends Model
+abstract class ModelWithSlug extends Model
 {
     protected static string $slugSource;