public function logInAsUserByUsername($username) {
		$numberOfMatchedUsers = $this->logInAsUserByColumnValue('username', \trim($username));

		if ($numberOfMatchedUsers === 0) {
			throw new UnknownUsernameException();
		}
		elseif ($numberOfMatchedUsers > 1) {
			throw new AmbiguousUsernameException();
		}
	}