php - How to display many to one relation objects in index action

I'm trying to create a symfony based forum. on the index page I need to show users sections they can choose to enter. however each section has categories, and all of those need to be displayed in one view. it should look like this.

• Section1
  • category1
  • category2
• Section2
  • category3

etc.

I'm following symblog.co.uk tutorial, and try to do it basing on their blogs/comments example, there is one simple problem, they define comments in show action, have $comments variable for each blog, and I need my categories to be accessible from $sections variable. For each Section user must be able to read categories AND add new ones

here's how my files look like.

index action, basic view where everything I wrote about takes place

{% block body %}
  IndexAction of Page Controller


  <form action="{{ path("EpiForumBundle_section_create") }}" method="post" {{ form_enctype(form) }} >
    {{ form_errors(form) }}

    {{ form_row(form.name) }}
        {{ form_rest(form) }}
    <input type="submit" value="Submit" />
  </form>

    <table>
      <th>date created
      <th>name
  {% for section in sections %}
        <tr>
          <td>
      <div ><time datetime="{{ section.created|date('c') }}">{{ section.created|date('l, F j, Y') }}</time></div>
          </td>
          <td>  
        <p><span >{{ section.name }}</span></p>
          </td>
        </tr>
  {% else %}
    <p>There are no sections for this forum</p>
  {% endfor %}
  </table>
{% endblock %}
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Page Controller

<?php

namespace Epi\ForumBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Epi\ForumBundle\Entity\Section;
use Epi\ForumBundle\Form\SectionType;

class PageController extends Controller
{
    public function indexAction()
    {       
            $section = new Section();
                $form = $this->createForm(new SectionType(), $section);

        $em = $this->getDoctrine()
                   ->getEntityManager();

        $sections = $em->getRepository('EpiForumBundle:Section')
                    ->getLatestSections();

        return $this->render('EpiForumBundle:Page:index.html.twig', array('sections' => $sections, 'form' => $form->createView()));
    }
}
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Section Controller

<?php

namespace Epi\ForumBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Epi\ForumBundle\Entity\Section;
use Epi\ForumBundle\Form\SectionType;

class SectionController extends Controller
{
    public function createAction()
    {
        $section = new Section();
            $form = $this->createForm(new SectionType(), $section);
            $em = $this->getDoctrine()->getManager();

            $request = $this->getRequest();

            if($request->getMethod() == 'POST'){
                $form->bind($this->getRequest());/*or  $form->handleRequest($request); depends on symfony version */
                $em->persist($section);
                $em->flush();
                return $this->redirect("/");
            }
            return $this->redirect("/");
    }
}
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Section Type:

<?php

namespace Epi\ForumBundle\Form;

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

class SectionType extends AbstractType
{
        /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('name');
    }

    /**
     * @param OptionsResolverInterface $resolver
     */
    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'Epi\ForumBundle\Entity\Section'
        ));
    }

    /**
     * @return string
     */
    public function getName()
    {
        return 'epi_forumbundle_section';
    }
}
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So now where do I put those forms for categories? How those forms should be written to know about section_id?