Quick auto-complete example

form.html.twig

{# just render your form like normal #}

{# here is some example JavaScript #}

{# I'm using https://select2.github.io/ - it's version 3.5.2, so the below code will probably need to be updated for version 4! #}
<script type="text/javascript" src="{{ asset('assets/vendor/select2/select2.min.js') }}"></script>
<script type="text/javascript">
    jQuery(document).ready(function() {
        $('.js-autocomplete-scientists').select2({
            placeholder: "Search some scientists",
            minimumInputLength: 1,
            multiple: true,
            ajax: {
                url: '{{ path('api_genus_scientist') }}',
                dataType: 'json',
                data: function (term, page) {
                    return {
                        q: term, // search term
                        page: page
                    };
                },
                results: function (data, page) {
                    return {results: data.scientists};
                }
            },
            initSelection: function(element, callback) {
                // get an array of the ids
                var ids = [];
                $(element.val().split(",")).each(function () {
                    ids.push(this);
                });

                // send an AJAX to get the user information
                $.ajax('{{ path('api_genus_scientist') }}', {
                    data: {
                        ids: ids
                    },
                    dataType: "json"
                }).done(function(data) {
                    // pass the array of users to the callback to continue
                    callback(data.scientists);
                });
            },
            formatResult: formatScientistResult,
            formatSelection: formatScientistSelection
        });
    });

    function formatScientistResult(scientist)
    {
        return '<div>'+scientist.username+'</div>';
    }

    function formatScientistSelection(scientist)
    {
        return scientist.username;
    }
</script>

HiddenMultipleEntityType.php

<?php

namespace Knp\UniversityBundle\Form\Type;

use Doctrine\Common\Persistence\ObjectManager;
use Knp\UniversityBundle\Form\DataTransformer\MultipleEntityToIdTransformer;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

/**
 * Based off of HiddenEntityType - for when you are selecting multiple items
 */
class HiddenMultipleEntityType extends AbstractType
{
    /**
     * @var ObjectManager
     */
    private $om;

    /**
     * @param ObjectManager $om
     */
    public function __construct(ObjectManager $om)
    {
        $this->om = $om;
    }

    /**
     * Add the data transformer to the field setting the entity repository
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $entityTransformer = new $options['transformer']($this->om);
        $entityTransformer->setEntityRepository($options['class']);
        $builder->addModelTransformer($entityTransformer);
    }

    /**
     * Require the entity repository option to be set on the field
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver
            ->setDefaults([
                'transformer' => MultipleEntityToIdTransformer::class,
            ])
            ->setRequired([
                'class',
            ])
        ;
    }

    /**
     * A text field, but your widget should hide this
     */
    public function getParent()
    {
        return TextType::class;
    }
}

services.yml

services:
    hidden_multiple_entity_type:
        class: Knp\UniversityBundle\Form\Type\HiddenMultipleEntityType
        arguments:
            - '@doctrine.orm.entity_manager'
        tags:
            - { name: form.type }

SomeController.php

<?php
// inside the controller, create the API action for auto-complete
public function listJsonAction(Request $request)
{
    $q = $request->query->get('q');
    if ($q) {
        $users = $this->getDoctrine()->getManager()
            ->getRepository('AppBundle:User')->searchByUsername($q);
    } elseif ($ids = $request->query->get('ids')) {
        $users = $this->getDoctrine()->getManager()
            ->getRepository('AppBundle:User')->findByIds($ids);
    } else {
        throw new BadRequestHttpException('Invalid filtering parameters');
    }

    $data = array('scientists' => array());
    foreach ($users as $user) {
        $data['scientists'][] = array(
            'id' => $user->getId(),
            'username' => $user->getUsername(),
        );
    }

    return new JsonResponse($data);
}

SomeFormType.php

// inside buildForm()

$builder
    ->add('authors', HiddenMultipleEntityType::class, array(
        'class' => 'AppBundle:User',
        'attr' => ['class' => 'js-autocomplete-scientists']
    ))
;