5c: Add the node as a constructor argument

> Undefined variable: node

web/modules/custom/my_module/src/Entity/Post.php

@@ -2,8 +2,16 @@
 
 namespace Drupal\my_module\Entity;
 
+use Drupal\node\NodeInterface;
+
 class Post {
 
+  private $node;
+
+  public function __construct(NodeInterface $node) {
+    $this->node = $node;
+  }
+
   public function getTitle(): string {
     return '';
   }