Controller\DemoController.php

<?php

namespace Acme\DemoBundle\Controller;

use Acme\DemoBundle\Form\Entity\Item;
use Acme\DemoBundle\Form\Entity\Tag;
use Acme\DemoBundle\Form\Type\ItemWithTagsType;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Symfony\Component\HttpFoundation\Request;
use Acme\DemoBundle\Form\ContactType;

// these import the "@Route" and "@Template" annotations
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Template;

class DemoController extends Controller
{
    /**
     * @Route("/contact", name="_demo_contact")
     * @Template()
     */
    public function contactAction(Request $request)
    {
        $data = new Item();
        $tag1 = new Tag();
        $tag1->name = 'PHP';
        $tag2 = new Tag();
        $tag2->name = 'Ruby';
        $data->tags = [$tag1, $tag2];

        $form = $this->createForm(new ItemWithTagsType(), $data);
        $form->handleRequest($request);

        if ($form->isValid()) {

            var_dump($form->getData());
        }

        return array('form' => $form->createView());
    }
}

Form\Entity\Item.php

<?php
namespace Acme\DemoBundle\Form\Entity;

use Symfony\Component\Validator\Constraints As Assert;

class Item
{
    public $id;

    /**
     * @Assert\NotBlank
     */
    public $name;

    /**
     * @assert\Valid
     */
    public $tags = [];
}

Form\Entity\Tag.php

<?php
namespace Acme\DemoBundle\Form\Entity;

use Symfony\Component\Validator\Constraints As Assert;

class Tag
{
    public $id;

    /**
     * @Assert\Length(max=10)
     */
    public $name;

    public function __construct($id = null, $name = null)
    {
        $this->id = $id;
        $this->name = $name;
    }
}

Form\Transformer\TagsToTextTransformer.php

<?php
namespace Acme\DemoBundle\Form\Transformer;

use Acme\DemoBundle\Form\Entity\Tag;
use Acme\DemoBundle\Repository\TagRepository;
use Symfony\Component\Form\DataTransformerInterface;

class TagsToTextTransformer implements DataTransformerInterface
{
    private $tagRepository;

    public function __construct(TagRepository $tagRepository)
    {
        $this->tagRepository = $tagRepository;
    }

    public function transform($value)
    {
        return array_reduce($value, function ($current, $element) {
            if ($current != '') {
                $current .= ' ';
            }

            return $current . $element->name;
        }, '');
    }

    public function reverseTransform($value)
    {
        $newTags = explode(' ', $value);

        $allTags = $this->tagRepository->findAll();
        $existingTags = array_filter($allTags, function ($element) use (&$newTags) {
            if (($foundIndex = array_search($element->name, $newTags)) !== false) {
                array_splice($newTags, $foundIndex, 1);
            }

            return $foundIndex !== false;
        });

        $tags = array_reduce($newTags, function ($current, $element) {
            $current[] = new Tag(null, $element);

            return $current;
        }, $existingTags);

        return $tags;
    }
}

Form\Type\ItemWithTagsType.php

<?php

namespace Acme\DemoBundle\Form\Type;

use Acme\DemoBundle\Form\Entity\Item;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class ItemWithTagsType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('name', 'text')
            ->add('tags', 'tags'])
            ->add('submit', 'submit')
        ;
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(
            [
                'data_class' => Item::class,
            ]
        );
    }

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

Form\Type\TagsType.php

<?php

namespace Acme\DemoBundle\Form\Type;

use Acme\DemoBundle\Form\Transformer\TagsToTextTransformer;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;

class TagsType extends AbstractType
{
    private $transformer;

    public function setTagsToTextTransformer(TagsToTextTransformer $transformer)
    {
        $this->transformer = $transformer;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->addModelTransformer($this->transformer);
    }

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

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

Repository\TagRepository.php

<?php
namespace Acme\DemoBundle\Repository;

use Acme\DemoBundle\Form\Entity\Tag;

class TagRepository
{
    public function findAll()
    {
        return [
            new Tag(1, 'PHP'),
            new Tag(2, 'Ruby'),
            new Tag(3, 'Python'),
            new Tag(4, 'Java')
        ];
    }
}

Resources\config\services.yml

services:
  acme.demo.formtype.tags:
    class: Acme\DemoBundle\Form\Type\TagsType
    tags:
      - { name: form.type, alias: tags }
    calls:
      - [setTagsToTextTransformer, [@acme.demo.formtransformer.tagstotext]]

  acme.demo.repository.tag_repository:
    class: Acme\DemoBundle\Repository\TagRepository
    # for normal ORM repository see http://stackoverflow.com/questions/12223176/how-to-inject-a-repository-into-a-service-in-symfony2

  acme.demo.formtransformer.tagstotext:
    class: Acme\DemoBundle\Form\Transformer\TagsToTextTransformer
    arguments: [@acme.demo.repository.tag_repository]

Resources\views\Demo\contact.html.twig

{% extends "AcmeDemoBundle::layout.html.twig" %}

{% block title "Symfony - Contact form" %}

{% block content %}
    {{ form_start(form) }}
    {{ form_errors(form) }}

    {{ form_row(form.name) }}
    {{ form_row(form.tags) }}

    {{ form_rest(form) }}
{% endblock %}