IssueSelectorType.php

<?php

namespace Acme\TaskBundle\Form\Type;

use Acme\TaskBundle\Form\DataTransformer\IssueToNumberTransformerFactory;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class IssueSelectorType extends AbstractType
{
    private $factory;
    
    public function __construct(IssueToNumberTransformerFactory $factory)
    {
        $this->factory = $factory;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $transformer = $this->factory->create($options['om']);
        $builder->addModelTransformer($transformer);
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'invalid_message' => 'The selected issue does not exist',
            'om' => 'default'
        ));
    }

    public function getParent()
    {
        return 'text';
    }

    public function getName()
    {
        return 'issue_selector';
    }
}

IssueToNumberTransformer.php

<?php

namespace Acme\TaskBundle\Form\DataTransformer;

use Symfony\Component\Form\DataTransformerInterface;
use Symfony\Component\Form\Exception\TransformationFailedException;
use Doctrine\Common\Persistence\ObjectManager;
use Acme\TaskBundle\Entity\Issue;

class IssueToNumberTransformer implements DataTransformerInterface
{
    /**
     * @var ObjectManager
     */
    private $om;

    /**
     * @param ObjectManager $om
     */
    public function __construct(ObjectManager $om)
    {
        $this->om = $om;
    }

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

        return $issue->getNumber();
    }

    /**
     * Transforms a string (number) to an object (issue).
     *
     * @param  string $number
     *
     * @return Issue|null
     *
     * @throws TransformationFailedException if object (issue) is not found.
     */
    public function reverseTransform($number)
    {
        if (!$number) {
            return null;
        }

        $issue = $this->om
            ->getRepository('AcmeTaskBundle:Issue')
            ->findOneBy(array('number' => $number))
        ;

        if (null === $issue) {
            throw new TransformationFailedException(sprintf(
                'An issue with number "%s" does not exist!',
                $number
            ));
        }

        return $issue;
    }
}

IssueToNumberTransformerFactory.php

<?php

namespace Acme\TaskBundle\Form\DataTransformer;

use Symfony\Bridge\Doctrine\ManagerRegistry;

class IssueToNumberTransformerFactory
{
    /** @var ManagerRegistry */
    private $registry;

    public function __construct(ManagerRegistry $registry)
    {
        $this->registry = $registry;
    }

    public function create($om)
    {
        return new IssueToNumberTransformer($this->registry->getManager($om));
    }
}

TaskType.php

<?php

namespace Acme\TaskBundle\Form;

use Acme\TaskBundle\Form\DataTransformer\IssueToNumberTransformerFactory;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class TaskType extends AbstractType
{
    /** @var IssueToNumberTransformerFactory */
    private $factory;

    public function __construct(IssueToNumberTransformerFactory $factory)
    {
        $this->factory = $factory;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $transformer = $this->factory->create($options['om']);

        $builder->add(
            $builder->create('issue', 'text')
                ->addModelTransformer($transformer)
        );
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver
            ->setDefaults(array(
                'data_class' => 'Acme\TaskBundle\Entity\Task',
            ))
            ->setRequired(array('om'))
        ;
    }
}