Show the number of parts with this element and all its child elements in Admin pages

src/Entity/Base/AbstractStructuralDBElement.php

@@ -93,7 +93,7 @@ abstract class AbstractStructuralDBElement extends AttachmentContainingDBElement
      * @NoneOfItsChildren()
      * @Groups({"include_parents"})
      */
-    protected $parent;
+    protected $parent = null;
 
     /** @var string[] all names of all parent elements as a array of strings,
      *  the last array element is the name of the element itself
@@ -271,16 +271,17 @@ abstract class AbstractStructuralDBElement extends AttachmentContainingDBElement
      */
     public function getSubelements(): iterable
     {
-        return $this->children;
+        return $this->children ?? new ArrayCollection();
     }
 
     /**
+     * @see getSubelements()
      * @return Collection<static>|iterable
      * @psalm-return Collection<int, static>
      */
     public function getChildren(): iterable
     {
-        return $this->children;
+        return $this->getSubelements();
     }
 
     public function isNotSelectable(): bool

src/Repository/AbstractPartsContainingRepository.php

@@ -45,6 +45,23 @@ abstract class AbstractPartsContainingRepository extends StructuralDBElementRepo
      */
     abstract public function getPartsCount(object $element): int;
 
+    /**
+     * Returns the count of the parts associated with this element and all its children.
+     * Please be aware that this function is pretty slow on large trees!
+     * @param  AbstractPartsContainingDBElement  $element
+     * @return int
+     */
+    public function getPartsCountRecursive(AbstractPartsContainingDBElement $element): int
+    {
+        $count = $this->getPartsCount($element);
+
+        foreach ($element->getChildren() as $child) {
+            $count += $this->getPartsCountRecursive($child);
+        }
+
+        return $count;
+    }
+
     protected function getPartsByField(object $element, array $order_by, string $field_name): array
     {
         if (!$element instanceof AbstractPartsContainingDBElement) {