Example. Related elements as select menu

======= ext_tables.sql ============
CREATE TABLE tx_extkey_domain_model_maintablename (
	partner int(11) unsigned DEFAULT '0'
);

======== TCA: your main TCA ===========
'partner' => array(
	'exclude' => 1,
	'label' => 'Partner',
	'config' => array(
		'type' => 'select',
		'renderType' => 'selectSingle',
		'foreign_table' => 'tx_extkey_domain_model_partner',
		'foreign_table_where' => 'ORDER BY tx_extkey_domain_model_partner.name'
	),
)

====== Model: your main model =========
/**
 * partner
 *
 * @var int
 */
protected $partner = 0;

/**
 * Returns the partner
 *
 * @return int
 */
public function getPartner()
{
	return $this->partner;
}

/**
 * Sets the partner
 *
 * @param int
 * @return void
 */
public function setPartner($partner)
{
	$this->partner = $partner;
}

===== Main Controller =======
/**
 * partnerRepository
 *
 * @var \Gigabonus\Gbpartner\Domain\Repository\PartnerRepository
 * @inject
 */
protected $partnerRepository = NULL;

/**
 * action new
 *
 * @return void
 */
public function newAction()
{
	$partners = $this->partnerRepository->findAll();
	$this->view->assign('partners', $partners);
}

========== view ===========
<select name="tx_gbaccount_transactions[newTransaction][partner]">
	<f:for each="{partners}" as="partner" iteration="Iteration">
		<option value="{partner.uid}">{partner.name}</option>
	</f:for>
</select>

========= better practice For storagePid ==========
plugin.tx_mainkey_pluginkey {
	settings {
		storagePid = {$plugin.tx_mainkey_pluginkey.persistence.storagePid}
	}
}

========= Controller again ==========
/**
*
* Property Mapping
*/
protected function initializeCreateAction(){
	$propertyMappingConfiguration = $this->arguments['newTransaction']->getPropertyMappingConfiguration();
	$propertyMappingConfiguration->allowAllProperties();
	$propertyMappingConfiguration->setTypeConverterOption('TYPO3\CMS\Extbase\Property\TypeConverter\PersistentObjectConverter', \TYPO3\CMS\Extbase\Property\TypeConverter\PersistentObjectConverter::CONFIGURATION_CREATION_ALLOWED, TRUE);
	$propertyMappingConfiguration->setTargetTypeForSubProperty('partner', 'int');
}

/**
 * action create
 *
 * @param \Gigabonus\Gbpartner\Domain\Model\Transaction $newTransaction
 * @return void
 */
public function createAction(\Gigabonus\Gbpartner\Domain\Model\Transaction $newTransaction)
{
	$settings = $this->settings;
	$pid = (int)$this->settings['storagePid'];
	$newTransaction->setPid($pid);
	$this->transactionRepository->add($newTransaction);
	$this->redirect('list');
}