A filter that allows you to sort by related entities using DunglasApiBundle (https://github.com/dunglas/DunglasApiBundle).

OrderByRelationFilter.php

<?php

namespace Your\AppBundle\Api\Filter;

use Dunglas\ApiBundle\Doctrine\Orm\Filter\AbstractFilter;

use Doctrine\Common\Persistence\ManagerRegistry;
use Doctrine\ORM\QueryBuilder;
use Dunglas\ApiBundle\Api\IriConverterInterface;
use Dunglas\ApiBundle\Api\ResourceInterface;

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\PropertyAccess\PropertyAccessorInterface;

/**
 * Description of OrderByRelationFilter
 *
 * @author thomas.kekeisen
 */
class OrderByRelationFilter extends AbstractFilter
{
    /**
     * @var array List of properties by witch the collection can or cannot be ordered.
     */
    protected $properties;

    /**
     * @var string Keyword used to retrieve the value.
     */
    protected $orderParameter;

    /**
     * @param ManagerRegistry $managerRegistry
     * @param string          $orderParameter  Keyword used to retrieve the value.
     * @param array|null      $properties      List of property names on which the filter will be enabled.
     */
    public function __construct(ManagerRegistry $managerRegistry, $orderParameter)
    {
        $this->managerRegistry = $managerRegistry;
        $this->orderParameter = $orderParameter;
    }

    /**
     * {@inheritdoc}
     */
    public function apply(ResourceInterface $resource, QueryBuilder $queryBuilder, Request $request)
    {
        $this->applyFilter($resource, $queryBuilder, $request->query->get($this->orderParameter));
    }

    /**
     * {@inheritdoc}
     */
    protected function applyFilter(ResourceInterface $resource, QueryBuilder $queryBuilder, array $values = null)
    {
        $metadata     = $this->getClassMetadata($resource);
        $associations = $metadata->getAssociationNames();
        $currentChar  = 65;

        if (!empty($values) && is_array($values))
        {
            foreach ($values as $relationName => $order)
            {
                $splittedName = explode('.', $relationName);

                if (count($splittedName) == 2)
                {
                    if (in_array($splittedName[0], $associations))
                    {
                        $mapping = $metadata->associationMappings[$splittedName[0]];

                        if ($mapping['type'] == '1' || $mapping['type'] == '2')
                        {
                            $class = $metadata->getAssociationTargetClass($splittedName[0]);

                            if (property_exists($class, $splittedName[1]))
                            {
                                $alias = 'oj'.chr($currentChar);

                                $queryBuilder
                                    ->leftJoin('o.'.$splittedName[0], $alias)
                                    ->addOrderBy(sprintf('%s.%s', $alias, $splittedName[1]), $order);
                            }

                            ++$currentChar;
                        }
                    }
                }
            }
        }
    }

    /**
     * {@inheritdoc}
     */
    public function getDescription(ResourceInterface $resource)
    {
        $description = [];

        // TODO: Return all possible sorting parameters?

        return $description;
    }
}