diff --git a/htdocs/core/class/commonobject.class.php b/htdocs/core/class/commonobject.class.php
index f11548d6a5a..c0fc7805bec 100644
--- a/htdocs/core/class/commonobject.class.php
+++ b/htdocs/core/class/commonobject.class.php
@@ -86,6 +86,11 @@ abstract class CommonObject
 	 */
 	public $element;
 
+	/**
+	 * @var int The related element
+	 */
+	public $fk_element;
+
 	/**
 	 * @var string 	Name to use for 'features' parameter to check module permissions user->rights->feature with restrictedArea().
 	 * 				Undefined means same value than $element. Can be use to force a check on another element for example for class of line, we mention here the parent element.