Fix parent nav id assignment in NavItem model

CHANGELOG.md

@@ -6,6 +6,7 @@ In order to read more about upgrading and BC breaks have a look at the [UPGRADE
 ## 5.1.1
 
 + [#410](https://github.com/luyadev/luya-module-cms/pull/410) Disabled sorting functionality for the "group" extra field in the block CRUD interface due to an exception being thrown. This issue occurred because the field is declared as an `extraAttribute`.
++ [#409](https://github.com/luyadev/luya-module-cms/issues/409) Implemented a new validation check to prevent slug duplication within the same language and navigation hierarchy when creating a new page. This enhancement ensures unique page identification and avoids conflicts in site structure.
 
 ## 5.1.0 (7. February 2024)
 

src/models/NavItem.php

@@ -256,14 +256,17 @@ public function verifyAlias($alias, $langId)
  return false;
  }
 
+ // when no parent nav id is given, the post value is `null` therefore we have the explicit set the value to `0`.
+ $parentNavId = $this->parent_nav_id ?: 0;
+
  /**
  * Group by website_id
  * @since 4.0.0
  */
  $exists = static::find()
  ->leftJoin('cms_nav', 'cms_nav_item.nav_id=cms_nav.id')
  ->leftJoin('cms_nav_container', 'cms_nav.nav_container_id=cms_nav_container.id')
- ->where(['cms_nav_item.alias' => $alias, 'cms_nav_item.lang_id' => $langId, 'cms_nav.parent_nav_id' => $this->parent_nav_id])
+ ->where(['cms_nav_item.alias' => $alias, 'cms_nav_item.lang_id' => $langId, 'cms_nav.parent_nav_id' => $parentNavId])
  ->groupBy('cms_nav_container.website_id')
  ->exists();
  if ($exists) {