protected function getUserMapper()
    {
        if ( null === $this->_userMapper )
        {
            $mapper = $this->getMapper();

            $this->_userMapper = $this->getServiceLocator()
                                      ->get( 'Di' )
                                      ->get( 'Grid\User\Model\User\Mapper', array(
                                          'dbAdapter' => $mapper->getDbAdapter(),
                                          'dbSchema'  => $mapper->getDbSchema(),
                                      ) );
        }

        return $this->_userMapper;
    }