Entity mapping in a Symfony2 choice field with optgroup - forms

Question

Suppose to have an entity in Symfony2 that has a field bestfriend, which is a User entity selected from a list of User entities that satisfy a complex requirement.
You can render this field in a form by specifying that it is an entity field type, i.e.:
$builder->add('bestfriend', 'entity', array(
'class' => 'AcmeHelloBundle:User',
'property' => 'username',
));
This form field is rendered as a <select>, where each one of the displayed values is in the form:
<option value="user_id">user_username</option>
So, one would render the field by using the <optgroup> tags to highlight such special feature of the friends.
Following this principle, I created a field type, namely FriendType, that creates the array of choices as in this answer, which is rendered as follows:
$builder->add('bestfriend', new FriendType(...));
The FriendType class creates a <select> organized with the same <option>s but organized under <optgroup>s.
Here I come to the problem! When submitting the form, the framework recognize that the user field is not an instance of User, but it is an integer. How can I let Symfony2 understand that the passed int is the id of an entity of type User?

Answer 1

Here follows my solution.
Notice that it is not mentioned in the Symfony2 official docs, but it works! I exploited the fact that the entity field type is child of choice.
Hence, you can just pass the array of choices as a param.
$builder->add('bestfriend', 'entity', array(
'class' => 'AcmeHelloBundle:User',
'choices' => $this->getArrayOfEntities()
));
where the function getArrayOfEntities() is a function that fills the choice list with the friends of my friends, organized by my friends:
private function getArrayOfEntities(){
$repo = $this->em->getRepository('AcmeHelloBundle:User');
$friends = $repo->findAllFriendByComplexCriteria(...);
$list = array();
foreach($friends as $friend){
$name = $friend->getUsername();
if(count($friend->getFriends())>0){
$list[$name] = array();
foreach($friend->getFriends() as $ff){
$list[$name][$ff->getUsername()] = $ff;
}
}
}
return $list;
}
I know the example could be meaningless, but it works...
PS: You need to pass the entity manager to let it working...