Symfony2 formtype how to pass data

I would like to build a form which contains data from 2 related entities and is able to update both entities with one submit. So far, I got a working form which uses formtype but only with static data (e.g. $foo = 'bar'). I don't get it how to pass the data from my controller to the formtype and using it in my formtype...

This is what I've got:

    // AdminController.php
    // throws exception
    $em = $this->getDoctrine()->getManager();

    $article = $em->getRepository('AcmeBlogBundle:Articles')->find($id);

    $form = $this->createForm(new ArticlesType(), $article);

This throws an exception:

    Expected argument of type "array or (\Traversable and \ArrayAccess)", "string" given 

So, I tried doing this

   //this is working, but (of course) empty
   $article = new Articles();

   $form = $this->createForm(new ArticlesType(), $article);

By the way this is my ArticlesType.php

class ArticlesType extends AbstractType {

    public function buildForm(FormBuilderInterface $builder, array $options)
    {

    $builder->add('headline', 'text', 
                array('label' => false))
            ->add('articleContent', 'textarea')
            ->add('state', 'entity', 
              array('class' => 'Acme\BlogBundle\Entity\Status',
                'property' => 'status',
                'label' => false))
            ->add('category', 'entity', 
              array('class' => 'Acme\BlogBundle\Entity\Categories',
                'property' => 'cat_name',
                'label' => false))
            ->add('save', 'submit')
            ->add('aufstellung', 'collection', array('type' => new AufstellungType()));

     }

     public function setDefaultOptions(OptionsResolverInterface $resolver)
     {
        $resolver->setDefaults(array(
        'data_class' => 'Acme\BlogBundle\Entity\Articles',
      ));
     }

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