diff --git a/app/library/Mvc/Model/Behavior/RandomId.php b/app/library/Mvc/Model/Behavior/RandomId.php
index e5c644a..93d3c2a 100644
--- a/app/library/Mvc/Model/Behavior/RandomId.php
+++ b/app/library/Mvc/Model/Behavior/RandomId.php
@@ -60,6 +60,12 @@ class RandomId extends Behavior implements BehaviorInterface
         $field = $this->_options['field'];
         $len = $this->_options['length'];
 
+        if (isset($this->_options['expression'])) {
+            $expr = 'AND ' . $this->_options['expression'];
+        } else {
+            $expr = '';
+        }
+
         if ($model->$field === null) {
 
             $random = new \Phalcon\Security\Random();
@@ -67,7 +73,7 @@ class RandomId extends Behavior implements BehaviorInterface
                 $id = substr($random->base64Safe(), 0, $len);
 
                 $count = $model->count(array(
-                    "$field = ?0",
+                    "$field = ?0 $expr",
                     'bind' => array($id)
                 ));