Add a default role during user registration with FOSUserBundle

I think @RayOnAir solution is right way of doing this. But it will not work due to FOS default role handling

to make possible to persist default role in database one need to override User::setRoles() method (add it to your User entity):

/**
 * Overriding Fos User class due to impossible to set default role ROLE_USER 
 * @see User at line 138
 * @link https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Model/User.php#L138
 * {@inheritdoc}
 */
public function addRole($role)
{
    $role = strtoupper($role);

    if (!in_array($role, $this->roles, true)) {
        $this->roles[] = $role;
    }

    return $this;
}

Tested under:

Symfony version 2.3.6, FOSUserBundle 2.0.x-dev

The recommended way to do it as indicated by a main contributor to the FOSUserBundle (in the comment here linked) is to register an Event Listener on the REGISTRATION_SUCCESS event and use the $event->getForm()->getData() to access the user and modify it. Following those guidelines, I created the following listener (which works!):

<?php

// src/Acme/DemoBundle/EventListener/RegistrationListener.php

namespace Acme\DemoBundle\EventListener;

use FOS\UserBundle\FOSUserEvents;
use FOS\UserBundle\Event\FormEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Listener responsible for adding the default user role at registration
 */
class RegistrationListener implements EventSubscriberInterface
{
    public static function getSubscribedEvents()
    {
        return array(
            FOSUserEvents::REGISTRATION_SUCCESS => 'onRegistrationSuccess',
        );
    }

    public function onRegistrationSuccess(FormEvent $event)
    {
        $rolesArr = array('ROLE_USER');

        /** @var $user \FOS\UserBundle\Model\UserInterface */
        $user = $event->getForm()->getData();
        $user->setRoles($rolesArr);
    }
}

Also, the service needs to be registered as follows:

// src/Acme/DemoBundle/Resources/config/services.yml
services:
    demo_user.registration_listener:
        class: Acme\DemoBundle\EventListener\RegistrationListener
        arguments: []
        tags:
            - { name: kernel.event_subscriber }

Notice that adding a default role in the User class __construct() may have some issues as indicated in this other answer.

What i have done is override the entity constructor:

Here a piece of my Entity/User.php

public function __construct()
{
    parent::__construct();
    // your own logic
    $this->roles = array('ROLE_USER');
}

This is the lazy way. If you want the right and better way see the @RayOnAir answer