symfony2 form choice and mongodb

Question

When using Entity we can load choices from Entity via form field type entity

$builder->add('group', 'entity', array(
    'class' => 'Fist\CoreBundle\Document\Group',
));  

but when using mongodb document what is the best way to inject select options from Document?

in my case load all Groups from Group document. Is there better way to do that than passing to form constructor?

Thank you all

Answer 1

I had to partially stray from Dylan Oliver's answer.

Symfony\Bundle\DoctrineMongoDBBundle\Form\Type\DocumentType

could not be found, as of November 2016 DocumentType seems to reside here:

Doctrine\Bundle\MongoDBBundle\Form\Type\DocumentType