Add: invalidDiscriminatorValue method

lib/Doctrine/ORM/Internal/Hydration/HydrationException.php

@@ -88,4 +88,18 @@ class HydrationException extends \Doctrine\ORM\ORMException
             $discrColumnName, $entityName, $dqlAlias
         ));
     }
+    
+    /**
+     * @param string $discrValue
+     * @param array  $discrMap
+     *
+     * @return HydrationException
+     */
+    public static function invalidDiscriminatorValue($discrValue, $discrMap)
+    {
+        return new self(sprintf(
+            'The discriminator value "%s" is invalid. It must be one of "%s".',
+            $discrValue, implode('", "', $discrMap)
+        ));
+    }
 }