namespace Application\Form;

use DoctrineModule\Validator\NoObjectExists as NoObjectExistsValidator;
use Zend\Form\Form;
use Zend\ServiceManager\ServiceManager;

class User extends Form
{
    public function __construct(ServiceManager $serviceManager)
    {
        parent::__construct('my-form');

        // Add an element
        $this->add(array(php
            'type'    => 'Zend\Form\Element\Email',
            'name'    => 'email',
            'options' => array(
                'label' => 'Email'
            ),
            'attributes' => array(
                'required'  => 'required'
            )
        ));

        // add other elements (submit, CSRF…)

        // Fetch any valid object manager from the Service manager (here, an entity manager)
        $entityManager = $serviceManager->get('Doctrine\ORM\EntityManager');

        // Now get the input filter of the form, and add the validator to the email input
        $emailInput = $this->getInputFilter()->get('email');

        $noObjectExistsValidator = new NoObjectExistsValidator(array(
            'object_repository' => $entityManager->getRepository('Application\Entity\User'),
            'fields'            => 'email'
        ));

        $emailInput->getValidatorChain()
                      ->addValidator($noObjectExistsValidator);
    }
}