Symfony2: Form should show Entity by Field related to another table

I have a form which should show all entries in a table, that is related to another table.

controller:

  public function maintainProjectAction($id, Request $request)
   {
    $em = $this->getDoctrine()->getManager();    
    $project = $em->getRepository('PsoProjectBundle:Project')->find($id);
    $form =  $this->createForm(new MaintainProjectType(),$project);
    $form->handleRequest($request);

    $article = $em->getRepository('PsoProjectBundle:Project')->findOneByProjects($id);
    $form2 =  $this->createForm(new ArticleType(),$article);
    $form2->handleRequest($request);        
    }

Entity:

 // src/Pso/ProjectBundle/Entity/Article.php
   namespace Pso\ProjectBundle\Entity;

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


/**
* @ORM\Entity
* @ORM\Table(name="article", options={"collate"="utf8_general_ci",    "charset"="utf8", "engine"="InnoDB"})
*/
class Article

{ 
/**
 * @ORM\Column(name="id", type="integer")
 * @ORM\Id()
 * @ORM\GeneratedValue(strategy="AUTO")
 */
 private $id;

/**
 * @ORM\ManyToOne(targetEntity="Pso\ProjectBundle\Entity\Project", inversedBy="article")
 * @ORM\JoinColumn(name="project_id", referencedColumnName="id")
 */
 private $projects;

 /**
 * @ORM\ManyToOne(targetEntity="Pso\LogBundle\Entity\User", inversedBy="article")
 * @ORM\JoinColumn(name="user_id", referencedColumnName="id")
 */
 private $users;


/**
 * @ORM\Column(type="string", length=50, nullable=false)
 */ 
private $category;

/**
 * @ORM\Column(type="blob", nullable=true)
 */
private $product_description;

/**
 * @ORM\Column(type="string", length=20, nullable=true)
 */ 
private $article_nr;


/**
 * @ORM\Column(type="string", length=30, nullable=true)
 */ 
private $color;

/**
 * @ORM\Column(type="integer", length=20, nullable=true)
 */
 private $carton_qty;

 /**
 * @ORM\Column(type="integer", length=20, nullable=true)
 */
 private $qty;

 /**
 * @ORM\Column(type="integer", length=20, nullable=true)
 */
 private $carton;

 /**
 * @ORM\Column(type="string", length=50, nullable=true)
 */ 
 private $size;

 /**
 * @ORM\Column(type="float", length=2, nullable=true)
 */ 
 private $price;

 /**
 * @ORM\Column(type="string", length=255, nullable=true)
 */ 
private $price_calculation_file;

/**
 * @ORM\Column(type="smallint", length=1)
 */
 private $closed;


 /**
 * @ORM\Column(type="datetime", nullable=false)
 */   
 private $dbinsert;


  /**
 * @ORM\Column(type="datetime", nullable=false)
 */ 
 private $dbupdate;

 /**
 * @ORM\Column(type="string", length=255, nullable=true)
 */ 
private $product_packaging;

/**
 * @ORM\Column(type="string", length=255, nullable=true)
 */ 
private $outer_packaging;
 }

The problem is, that the Formtype should show all entries, where $projects = $id

All things I tried, didn't work.

I tried to get it work also inside the formbuilder with

->add('id','entity', 'hidden', array(
                                                    'label' => 'ID',
                                                    'class' => 'Pso\ProjectBundle\Entity\Article',
                                                    'property' => 'id',
                                                    'query_builder'  => function(EntityRepository $er) {
                                                        return $er->createQueryBuilder('a')
                                                            ->FROM('PsoProjectBundle:article a')
                                                            ->JOIN('a.projects p')
                                                            ->WHERE('p.id = :id')
                                                            ->setParameter('id', $id);
                                                        },
                                                    ))

The goal is, that in the form sould be shown all articles, that belong to the project-ID in the controller.