Webservice/Form/Transformer/ClientTransformer.php

<?php
namespace WebserviceBundle\Form\Transformer;

use Doctrine\ORM\EntityManagerInterface;
use Symfony\Component\Form\DataTransformerInterface;
use Symfony\Component\Form\Exception\TransformationFailedException;

class ClientTransformer implements DataTransformerInterface
{
    private $em;

    public function __construct(EntityManagerInterface $em)
    {
        $this->em = $em;
    }

    /**
     * Transforms an object (issue) to a string (number).
     *
     * @param  Int | null $client
     * @return string
     */
    public function transform($client)
    {
        if (null === $client) {
            return '';
        }

        return $client;
    }

    /**
     * Transforms a string (query pattern) to an client id (using the web service).
     *
     * @param  string $client_query_param
     * @return Issue|null
     * @throws TransformationFailedException if object (issue) is not found.
     */
    public function reverseTransform($client_query_param)
    {
        // no issue number? It's optional, so that's ok
        if (!$issueNumber) {
            return;
        }

        $issue = $this->em
            ->getRepository(Issue::class)
            // query for the issue with this id
            ->find($issueNumber)
        ;

        if (null === $issue) {
            // causes a validation error
            // this message is not shown to the user
            // see the invalid_message option
            throw new TransformationFailedException(sprintf(
                'An Client could not be found for the query "%s"!',
                $issueNumber
            ));
        }

        return $issue;
    }
}