Symfony 2 Exception: Call to a member function on a non object when using Form Events

Question

I'm trying to create a form which dinamically load all "sites" related to a "project", It seems like this would be of use, so I tried it:

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;

class EngineeringType extends AbstractType
{
        /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('project','entity',array(
                'class' => 'tBundle:Project',
                'label' => 'Project'
            ;
    $builder->addEventListener(
            FormEvents::PRE_SET_DATA,
            function(FormEvent $event) {
                $form = $event->getForm();
                $data = $event->getData();

                $sites = $data->getProject()->getSites();

                $form->add('site', 'entity', array('choices' => $sites));
            }
        );
    }

My problem comes when I try to access the form, I get:

FatalErrorException: Error: Call to a member function getSites() on a non-object in ... tBundle\Form\EngineeringType.php line 41

Here are my entities:

namespace tBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * Engineering
 *
 * @ORM\Table(name="engineerings")
 * @ORM\Entity
 */
class Engineering
{
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */

    private $id;

    /**
      * @ORM\ManyToOne(targetEntity="tBundle\Entity\Project")
      * @ORM\JoinColumn(name="project_id", referencedColumnName="id",nullable=false)
      */
    private $project;

    /**
     * Get id
     *
     * @return integer 
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set project
     *
     * @param string $project
     * @return Engineering
     */
    public function setProject(\tBundle\Entity\Project $project)
    {
        $this->project = $project;

        return $this;
    }

    /**
     * Get project
     *
     * @return string 
     */
    public function getProject()
    {
        return $this->project;
    }

Project:

namespace tBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * Project
 *
 * @ORM\Table(name="projects")
 * @ORM\Entity
 */
class Project
{
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="name", type="string", length=255)
     */
    private $name;


     /**
     * @ORM\ManyToMany(targetEntity="tBundle\Entity\Site")
     * @ORM\JoinTable(name="project_sites",
     *      joinColumns={@ORM\JoinColumn(name="site_id", referencedColumnName="id")},
     *      inverseJoinColumns={@ORM\JoinColumn(name="project_id", referencedColumnName="id")}
     *      )
     */
    private $sites;

        public function __construct()
    {
        $this->sites = new ArrayCollection();
    }

    /**
     * Get id
     *
     * @return integer 
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set name
     *
     * @param string $name
     * @return Project
     */
    public function setName($name)
    {
        $this->name = $name;

        return $this;
    }

    /**
     * Get name
     *
     * @return string 
     */
    public function getName()
    {
        return $this->name;
    }

    /**
     * Get Sites
     *
     * @return array 
     */
    public function getSites()
    {
        return $this->sites;
    }

    /* Returns Project's Name */
    public function __toString() 
    {
        return $this->name;
    }

What am I doing wrong?

EDIT

Controller:

/**
    * Creates a form to create a Engineering entity.
    *
    * @param Engineering $entity The entity
    *
    * @return \Symfony\Component\Form\Form The form
    */
    private function createCreateForm(Engineering $entity)
    {
        $form = $this->createForm(new EngineeringType(), $entity, array(
            'action' => $this->generateUrl('engineering_create'),
            'method' => 'POST',
        ));

        $form->add('submit', 'submit', array('label' => 'Create'));

        return $form;
    }


 /**
    * Creates a form to edit a Engineering entity.
    *
    * @param Engineering $entity The entity
    *
    * @return \Symfony\Component\Form\Form The form
    */
    private function createEditForm(Engineering $entity)
    {
        $form = $this->createForm(new EngineeringType(), $entity, array(
            'action' => $this->generateUrl('engineering_update', array('id' => $entity->getId())),
            'method' => 'POST',
        ));

        $form->add('submit', 'submit', array('label' => 'Update'));

        return $form;
    }

Answer 1

The PRE_SET_DATA event is actually fired twice. The first time will not have any data. There used to be a blurb in the manual explaining why but I could not find it again.

So just:

        function(FormEvent $event) {
            $form = $event->getForm();
            $data = $event->getData();
            if ($data)
            {

                $sites = $data->getProject()->getSites();

                $form->add('site', 'entity', array('choices' => $sites));
            }
        }

=======================================================

Updated answer to show how to handle non-existent $project:

            if ($data)
            {
                $project = $data->getProject();
                $sites = $project ? $project->getSites() : array();

                $form->add('site', 'entity', array('choices' => $sites));
            }

Answer 2

There is an easy solution, why don't you use the "property" property on the form builder?

$builder
        ->add('project','entity',array(
            'class' => 'tBundle:Project',
            'label' => 'Project',
            'property' => 'sites');

Or even you can use a query builder if this is not enough:

$builder->add('users', 'entity', array(
    'class' => 'AcmeHelloBundle:User',
    'query_builder' => function(EntityRepository $er) {
        return $er->createQueryBuilder('u')
            ->orderBy('u.username', 'ASC');
    },
));

You can find here more description, if this is not enough: Symfony documentation

EDIT:

So your problem that with the first solution, that it's going to be an array, so use my second option, and in the query builder, specify what will reflect your needs. Or use it like the class is not Project but Sites.