how to: use Tx_Extbase_Domain_Repository_FrontendUserRepository in typo3 v4.5 - typo3

Question

I am trying to read the username of a front end user whose uid is known. I tried this in my controller's showAction method:
$objectManger = t3lib_div::makeInstance('Tx_Extbase_Object_ObjectManager');
// get repository
$repository = $objectManger->get('Tx_Extbase_Domain_Repository_FrontendUserRepository ');
$newObject = $repository->findByUid($coupon->getCreator()); //this is the uid of whoever was loggin in
print_r($newObject);
echo $newObject->getUsername(); die;
but when that code runs I get:
Oops, an error occured!
"Tx_Extbase_Domain_Repository_FrontendUserRepository " is not a valid cache entry identifier.
It turns out that $repository is empty, so how do I get fe_user data?
I am using typo3 v4.5 with extbase.
Thanks
Update to show complete answer.
This is the code (it goes in my CouponController) that worked (plus the typoscript mentioned):
/**
* #var Tx_Extbase_Domain_Repository_FrontendUserRepository
*/
protected $userRepository;
/**
* Inject the user repository
* #param Tx_Extbase_Domain_Repository_FrontendUserRepository $userRepository
* #return void */
public function injectFrontendUserRepository(Tx_Extbase_Domain_Repository_FrontendUserRepository $userRepository) {
$this->userRepository = $userRepository;
}
public function showAction(Tx_Coupons_Domain_Model_Coupon $coupon) {
$userRepository = $this->objectManager->get("Tx_Extbase_Domain_Repository_FrontendUserRepository");
$newObject = $userRepository->findByUid($coupon->getCreator());
$this->view->assign('coupon', $coupon);
$this->view->assign('creatorname', $newObject->getUsername() );
}

Answer 1

If you are using extbase yourself you dont have to call makeInstance for your objectManager, it's already there ($this->objectManager).
anyway, you should inject this repository (see my answer here: TYPO3 - Call another repository)
Clear the Cache after the Injection.
You maybe have to disable the recordtype extbase sets for its FrontendUser:
config.tx_extbase.persistence.classes.Tx_Extbase_Domain_Model_FrontendUser.mapping.recordType >

Answer 2

Set the source storage pid where the repository fetches the data from:
/** #var Tx_Extbase_Domain_Repository_FrontendUserRepository $repos */
$repos = $this->objectManager->get("Tx_Extbase_Domain_Repository_FrontendUserRepository");
$querySettings = $repos->createQuery()->getQuerySettings();
$querySettings->setStoragePageIds(array(123, 567));
$repos->setDefaultQuerySettings($querySettings);
$user = $repos->findByUid(56); // Queries for user 56 in storages 123 and 567