Dynamically remove Options from symfony2 choice (entity) form - symfony

Question

I have a Entity Type form where I list all the friends of the current user. This is e.g. used for creating a new Group. Now I want to use the same form for adding people to the same group, so the Choice field should show all the friends of the current user that are not yet in the group. I thought I just use a form event to remove the options(users) that are already in the group.
My listener looks like this:
class FriendSelectListener implements EventSubscriberInterface {
public static function getSubscribedEvents() {
// Tells the dispatcher that we want to listen on the form.pre_set_data
// event and that the preSetData method should be called.
return array(FormEvents::PRE_SET_DATA => 'preSetData');
}
public function preSetData(FormEvent $event) {
$betRound = $event->getData();
$form = $event->getForm();
$groupForm = $form->get('userGroup');
$usersForm = $groupForm->get('users');
foreach($betRound->getUserGroup()->getUsers() as $user){
if($usersForm->has($user->getId())){
$usersForm->remove($user->getId());
}
}
}
}
But I can't render it, because in my test I removed the user with the id 2 and then I get the following error message while rendering:
Key "2" in object (with ArrayAccess) of type
"Symfony\Component\Form\FormView" does not exist in
/var/lib/stickshift/1ed1210eec124c179c45aac4ad484afd/app-root/data/269891/src/Strego/UserBundle/Resources/views/Form/selectFriends.html.twig
at line 5
Update:
This might be related to my view:
{% for id, choice in choices %}
{% set user = choice.data %}
<label>
<div class="people-list people-choose unselected" style="width:270px;">
<img src="{{ user.profilePic | imagine_filter('profile_thumb') }}" class="img-polaroid" alt="{{ user.nickname }}">
{#<img src="{{ asset('bundles/stregoapp/img/profile-thumb-90x90.png') }}" alt="Profilbild" class="img-polaroid">#}
<p>{{ form_widget(form[id]) }} <span class="label">einladen</span></p>
<h3>{{ user.nickname }}</h3>
<h3><small>{{ user.firstName }} {{ user.lastName }}</small></h3>
</div>
</label>
{% endfor %}
For me it seems like I only removed the form element but not the choice.

Answer 1

I found the problem, it was indeed my view. As soon as I stopped iterating over the options but used the child elemts of my form element it was working:
{% for child in form %}
{% set user = choices[child.vars.value].data %}
<label>
<div class="people-list people-choose unselected" style="width:270px;">
<img src="{{ user.profilePic | imagine_filter('profile_thumb') }}" class="img-polaroid" alt="{{ user.nickname }}">
<p>{{ form_widget(child) }} <span class="label">einladen</span></p>
<h3>{{ user.nickname }}</h3>
<h3><small>{{ user.firstName }} {{ user.lastName }}</small></h3>
</div>
</label>
{% endfor %}