typo3 flow: variable in repository

I´d like to use a variable in a typo3 flow repository. With

$letter = $_POST['someVariable'];

it works with my following Repository:

public function findLetter() {

            $letter = $_POST['letter'];

            $query = $this->createQuery();
            $query->matching(
                    $query->like('name', $letter)
                    );
            return $query->execute();
}

I read that it should also be possible in typo3 flow to get variables by

$letter = $this->request->getArgument('someVariable');

but this doesn´t work for me; I get the following error:

#1: Notice: Undefined property: ......\Domain\Repository\MitgliedRepository::$request in /var/www/apps/flow/Data/Temporary/Development/Cache/Code/Flow_Object_Classes/..._..._Domain_Repository_...Repository.php line 96

line 96 in ...Repository is that:

$letter = $this->request->getArgument('letter');

Does anybody know, what I´m doing wrong?

I got it:

My Controller now looks linke that:

/**
 * @return void
 * @param string $letter
 */
public function letterAction($letter) {
    $this->view->assign('mitglieder', $this->mitgliedRepository->findLetter($letter));
}

And my Repository looks like that:

/**
 * @return string
 */
public function findLetter($letter) {
    $query = $this->createQuery();
    $query->matching(
        $query->like('name', $letter)
        )
        ->setOrderings(array('name' => \TYPO3\Flow\Persistence\QueryInterface::ORDER_ASCENDING));
    return $query->execute();
}