currently

1-controller.php

    public function createAction()
    {
        $service = $this->getService();
        $form = $service->getForm();
        $entity = $service->getEntity();

        $form->bind($entity);

        if($this->request->isPost()) {
            $mandator = $this->getEntityManager()->find('Admin\Entity\Mandator', $_POST['mandator_id']); //<--- should not be needed
            $_POST['mandator'] = $mandator;
            $form->setData($this->request->getPost());
            $this->getLogger()->info('Form Post', $_POST);
            if($form->isValid()) {
                $entity->setMandator($mandator); //<-- should not be needed
                $service->persist($entity);

                $this->getLogger()->info($entity->getName().' Successfully saved');
                $this->getMessenger()->setNamespace('ok')->addMessage('%s successfully saved');
                $this->redirect()->toUrl('/agency/list');
            }
        }

        return new ViewModel(array('form' => $form));
    }

2-form.php

<?php
namespace Admin\Form;

use Zend\InputFilter\InputFilterAwareInterface;

use Zend\Form\Element\Select;
use cwdAdmin\Form\DisplayGroup;
use cwdAdmin\Form\Form;

class Agency extends Form implements InputFilterAwareInterface {

    public function invoke()
    {

        parent::__construct('agency');
        $this->setAttribute('method', 'post');

        $this->addHidden('agency_id');

        $this->addText('name', 'Name', true);
        $this->addText('address', 'Address');
        $this->addText('address2', 'Address2');
        $this->addText('zipcode', 'Zipcode');
        $this->addText('city', 'City');
        $this->addText('uid', 'UID');
        $select = new Select('country');
        $select->setAttribute('type', 'select')
               ->setValueOptions($this->countryOptions())
               ->setLabel('Country');
        $this->add($select);

        $address = new DisplayGroup('fs_address', 'Address');
        $address->add('name')
                ->add('address')
                ->add('address2')
                ->add('zipcode')
                ->add('city')
                ->add('country')
                ->add('uid');
        $this->addDisplayGroup($address);

        $this->addText('phone', 'Phone');
        $this->addText('fax', 'Fax');
        $this->addText('url', 'URL');
        $this->addText('email', 'EMail');

        $contact = new DisplayGroup('fs_contact', 'Contact');
        $contact->add('phone')
                ->add('fax')
                ->add('url')
                ->add('email');
        $this->addDisplayGroup($contact);


        $select = new Select('mandator_id');
        $select->setAttribute('type', 'select')
               ->setValueOptions($this->getMandator())
               ->setLabel('Mandator');
        $this->add($select);

        $admin = new DisplayGroup('fs_admin', 'Administration');
        $admin->add('mandator_id');
        $this->addDisplayGroup($admin);
    }

    public function getMandator() {
        $em  = $this->getServiceLocator()->get('Doctrine\ORM\EntityManager');
        $rep = $em->getRepository('Admin\Entity\Mandator');

        $rows = $rep->findBy(array());
        $data = array();
        foreach($rows as $row) {
            $data[$row->getMandatorId()] = $row->getName();
        }

        return $data;
    }
}