diff --git a/doctrine-mapping.xsd b/doctrine-mapping.xsd
index 1f3f1476d..51dc089f9 100644
--- a/doctrine-mapping.xsd
+++ b/doctrine-mapping.xsd
@@ -218,9 +218,16 @@
 
   <xs:complexType name="order-by-field">
       <xs:attribute name="field" type="XS:NMTOKEN" use="required" />
-      <xs:attribute name="direction" type="XS:NMTOKEN" default="ASC" />
+      <xs:attribute name="direction" type="orm:order-by-direction" default="ASC" />
   </xs:complexType>
 
+  <xs:simpleType name="order-by-direction">
+    <xs:restriction base="xs:token">
+      <xs:enumeration value="ASC"/>
+      <xs:enumeration value="DESC"/>
+    </xs:restriction>
+  </xs:simpleType>
+
   <xs:complexType name="many-to-many">
     <xs:sequence>
       <xs:element name="cascade" type="orm:cascade-type" minOccurs="0" />