Build a form having a checkbox for each entity in a doctrine collection

I think this will answer your question.

Forget the FooEntitySelectionType. Add a property_path field option to FooEntitySelectByIdentityType and set the data_class option to null:

class FooEntitySelectByIdentityType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('foo_id', 'entity', array(
            'required'      => false,
            'class'         => 'MeMyBundle:FooEntity',
            'property'      => 'id',
            'property_path' => '[id]', # in square brackets!
            'multiple'      => true,
            'expanded'      => true
        ));
    }

    public function setDefaultOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults(array(
            'data_class'      => null,
            'csrf_protection' => false
        ));
    }

# ...

and in your controller, build the FooEntitySelectByIdentityType:

$form = $this
    ->createForm(
        new \Me\MyBundle\Form\Type\FooEntitySelectByIdentityType,
        $collection_of_foo
    )
    ->createView()
;

and then in the controller action which receives the POSTed data:

$form = $this
    ->createForm(new \Me\MyBundle\Form\Type\FooEntitySelectByIdentityType)
;
$form->bind($request);
if ($form->isValid()) {
    $data = $form->getData();
    $ids  = array();
    foreach ($data['foo_id'] as $entity) {
        $ids[] = $entity->getId();
    }
    $request->getSession()->set('admin/foo_list/batch', $ids);
}

and finally, in your twig template:

{# ... #}
{% for entity in foo_entity_collection %}
    {# ... #}

    {{ form_widget(form.foo_id[entity.id]) }}

    {# ... #}