diff --git a/src/Reference_One.php b/src/Reference_One.php
index <HASH>..<HASH> 100644
--- a/src/Reference_One.php
+++ b/src/Reference_One.php
@@ -87,6 +87,13 @@ class Reference_One extends Reference
     public $mandatory = false;
 
     /**
+     * Is field required? By default fields are not required.
+     *
+     * @var bool|string
+     */
+    public $required = false;
+
+    /**
      * Should we use typecasting when saving/loading data to/from persistence.
      *
      * Value can be array [$typecast_save_callback, $typecast_load_callback].
@@ -164,6 +171,7 @@ class Reference_One extends Reference
                 'caption'           => $this->caption,
                 'ui'                => $this->ui,
                 'mandatory'         => $this->mandatory,
+                'required'          => $this->required,
                 'typecast'          => $this->typecast,
                 'serialize'         => $this->serialize,
                 'persist_format'    => $this->persist_format,
Pass 'required' as we do with all other fields