Symfony textfield ajax users - php

Question

I have a Message entity and a User one. I want to make users be able to send messages to others. I already can send and receive messages, everything is working fine, but in order to select the receiver of a message, a default drop-down with all the users is made.
What I want to do is change the drop-down to a text field and make an AJAX to the database and retrieve a list of users that are found matching the keyword inserted in the textfield. And when clicking upon one of them, the user is introduced in the textfield.
The problem that I am facing is that I don't know how to pass an entity instead of plaintext because I get this error
Argument 1 passed to PrivateMessageBundle\Entity\Message::setReceiver() must be an instance of CrudBundle\Entity\User, string given
Another question, is there a bundle or plugin that does this easier and I can modify? Or do I have to do it myself everytime for every ajax field?
I'm building the form like so
$builder
->add('title')
->add('content','textarea')
->add('receiver','text');
By default, receiver is a drop-down select and automatically selects the User entity, but once I change it to text it doesn't.
Here's also relevant parts of my entities, if you need them:
class Message
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=50)
*/
protected $title;
/**
* #ORM\ManyToOne(targetEntity="CrudBundle\Entity\User")
* #ORM\JoinColumn(referencedColumnName="id")
*/
protected $receiver;
/**
* #ORM\ManyToOne(targetEntity="CrudBundle\Entity\User")
* #ORM\JoinColumn(referencedColumnName="id")
*/
protected $sender;
/**
* #var string
*
* #ORM\Column(name="content", type="string", length=2000)
*/
protected $content;
And my User class, extending FOSUserBundle's BaseUser class.
class User extends BaseUser implements AuthorInterface
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;

Answer 1

I think there are two ways to solve this problem.
1) add a custum formfield with the mapped = FALSE attribute.
->add('ajaxsearch', null, array('mapped' => false))
2) use a data-transformer.