diff --git a/docs/en/reference/association-mapping.rst b/docs/en/reference/association-mapping.rst
index 3a597f6db..f41bb610f 100644
--- a/docs/en/reference/association-mapping.rst
+++ b/docs/en/reference/association-mapping.rst
@@ -83,6 +83,7 @@ Generated MySQL Schema:
     CREATE TABLE Product (
         id INT AUTO_INCREMENT NOT NULL,
         shipping_id INT DEFAULT NULL,
+        UNIQUE INDEX UNIQ_6FBC94267FE4B2B (shipping_id),
         PRIMARY KEY(id)
     ) ENGINE = InnoDB;
     CREATE TABLE Shipping (