似乎无法扩展Member Object

Question

I'm trying to extend the Member class with a DataExtension as it recommends in the docs. I've managed to attach my DataExtension to the Member class however I cannot access my newly defined methods on the Member object.

MemberExtension.php

<?php

/** 
* Extends the standard SilverStripe Member DataObject 
*/ 
class MemberExtension extends DataExtension { 
   private static $db = array( 
      'MemberExpires' => 'Date', 
      'Username' => 'Varchar' 
   );

   /** 
    * Returns the Member Object for the username specified 
    * 
    * @param $id int the username 
    * @returns Member|null the Member object or null if not found 
    */ 
   public static function getByUsername(string $username) { 
      return Member::get()->filter('Username', $username)->first(); 
   } 
}

I'm adding the extension in the YAML config, I do a /dev/build and the new columns appear in the DB. When I run Object::get_extensions('Member') it properly returns an array with my MemberExtension class. However whenver I run Member::getByUsername() it always fails citing a Fatal error: Call to undefined method Member::getByUsername()

Am I doing this properly? Is their anything I'm not grasping?

Thanks for your time.