Change symfony2 form field but keep relation - forms

Question

So I have a form that is based on an entity that contains a one to many relationship.
The problem is that this field is rendered as a select (or choice). I really don't want to load all the possible ids (there are many) but just want to load the one that is set in the entity (which is the id that appears selected in the select).
Is there any way of doing this and still keep the relationship? If I really have to change the field how can I access, in the form class, the selected entity given to the entity so that I can retrieve the id?
UPDATE
To make this a bit clearer here is my form code:
$this->createFormBuilder()
->add('items', 'collection', array(
'type' => new \MyBundle\Form\ItemsType(),
'allow_add' => true,
'data' => $itemsEntities
)
)
->add('submit', 'submit')
In the $itemsEntities I have 5 entities all of which generate the select with loads of ids. Hakins answer would work I think if this would be just one field but since there are many I don't really know how to handle this.
I have tried to put an eventListner on the \MyBundle\Form\ItemsType for but I can never access any data.

Answer 1

Maybe you could use the 'query_builder' option of the field (see: http://symfony.com/doc/current/reference/forms/types/entity.html) and create a query that fetches the only result you want, based on it's id. You could pass the id to the constructor of form if necessary.

Answer 2

You can pass the id of the related entity to the form builder parameters, and change your field type to hidden instead of choice (or entity):
In your controller:
$id = $entity->getRelatedEntity()->getId();
$options['id'] = $id;
$form = $this->createForm(new EntityType($options), $entity);
In your EntityType:
public function buildForm(FormBuilderInterface $builder, array $options) {
$options = $this->options;
$builder
->add('relatedEntity', 'hidden', array(
'data' => $options['id'],
'required' => TRUE
));
Update
To avoid rendering a collection without changing the relationship, you can change only your Twig form by rendering the selected item(s) id(s) as hidden field(s), then set rendered the form.items. (If you don't set them rendered, they will be present in the form_rest(form))
With your existing code for the formBuilder, change your twig like this:
{% block body %}
...
{% for item in form.items %}
{% if item.vars.data %}
<input type="hidden" name="{{ item.vars.full_name}}" id="{{ item.vars.id }}" value="{{ item.vars.value }}"
{% endif %}
{% endfor %}
{% do form.items.setRendered %}
...
{% endblock %}