Symfony2 choice list type for MongoDB backed forms

A couple of days ago, I had to implement a select field representing a one-to-one MongoDB relationship in one of my Symfony2 forms. Having spent some time reading the documentation, I decided to use the entity form field type for this purpose.

class AcmeFormType extends AbstractType
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->add('parent', 'entity', array(
                'class' => 'Acme\MyBundle\Document\MyDocument',
                'property' => 'name',
                'label' => 'Parent',
                'empty_value' => 'Root'
            ));
     }
}

And I got presented with this exception:

Class Acme\MyBundle\Document\MyDocument is not a valid entity or mapped super class. 

I realized Symfony2 was trying to load the necessary class using the default entity manager defined by the Doctrine ORM and failing utterly. After digging through the code, I found out what I was looking for inside the vendor/bundles/Symfony/Bundle/DoctrineMongoDBBundle/Form/Type/DocumentType.php class. This class is the "entity" type equivalent for MongoDB backed forms. It's alias is document instead of entity, and it is registered into the service container with the usual tag/name/alias combination as can be seen in the service configuration XML snippet below.

src/vendor/bundles/Symfony/Bundle/DoctrineMongoDBBundle/Resources/config/mongodb.xml

<service id="form.type.mongodb_document" class="SymfonyBundleDoctrineMongoDBBundleFormTypeDocumentType">
    <tag name="form.type" alias="document" />
    <argument type="service" id="doctrine.odm.mongodb.document_manager" />
</service>

Long story short, if you are using MongoDB as your backend, your forms must use the document type for your choice lists.

class AcmeFormType extends AbstractType
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->add('parent', 'document', array(
                'class' => 'Acme\MyBundle\Document\MyDocument',
                'property' => 'name',
                'label' => 'Parent',
                'empty_value' => 'Root'
            ));
     }
}

The configuration for document choice lists is the same as entity choice lists. See http://symfony.com/doc/current/reference/forms/types/entity.html for more information.