The SortableNullsWalker is a TreeWalker that walks over a DQL AST and constructs the corresponding SQL to allow ORDER BY IS NULL http://devandco.wordpress.com/2013/11/23/doctrine-mysql-isnull/

SortableNullsWalker.php

<?php

namespace Application\DoctrineExtensions\Query\MySql;

use Doctrine\ORM\Query\SqlWalker;
use Doctrine\DBAL\DBALException;

/**
 * $query = $qb->getQuery();
 * $query->setHint(Query::HINT_CUSTOM_OUTPUT_WALKER, '\Namespace\DoctrineExtensions\Query\MySql\SortableNullsWalker');
 * $query->setHint("sortableNulls.fields", array("alias.field" => \Namespace\DoctrineExtensions\Query\MySql\SortableNullsWalker::NULLS_FIRST));
 *  
 * @see  http://stackoverflow.com/questions/12652034/how-can-i-order-by-null-in-dql
 * @see  http://stackoverflow.com/questions/2051602/mysql-orderby-a-number-nulls-last
 * @see  https://github.com/beberlei/DoctrineExtensions/blob/master/lib/DoctrineExtensions/Query/SortableNullsWalker.php
 */
class SortableNullsWalker extends SqlWalker
{
  const NULLS_FIRST = 'NULLS FIRST';
  const NULLS_LAST  = 'NULLS LAST';

  protected $sortableNullsFields;

 /**
  * Check if the platform is MySql
  * @param  OrderByClause $orderByClause
  * @return string
  */
  public function walkOrderByClause($orderByClause)
  {
    // Store sortable nulls fields
    $this->sortableNullsFields = $this->getQuery()->getHint('sortableNulls.fields');

    // MySql platform ?
    if (count($this->sortableNullsFields) 
      && $this->getConnection()->getDatabasePlatform()->getName() != 'mysql'
    )
    {
      throw DBALException::notSupported('SortableNullsWalker only supported by MySql');
    }

    return parent::walkOrderByClause($orderByClause);
  }

 /**
  * For each item we check if it's an isnull expression
  * @param  orderByItem $orderByItem
  * @return string
  */
  public function walkOrderByItem($orderByItem)
  {
    $expr = $orderByItem->expression;
    $key  = $expr->identificationVariable . '.' . $expr->field;

    if (isset($this->sortableNullsFields[$key])
      && $this->sortableNullsFields[$key] == self::NULLS_FIRST
    )
    {
      return sprintf(
        'ISNULL(%s), %s',
        $this->walkPathExpression($expr),
        parent::walkOrderByItem($orderByItem)
      );
    }

    return parent::walkOrderByItem($orderByItem);
  }
}