public function getReadPreference()
    {
        if (!is_object($this->_readPreference)) {
            if ($this->_readPreference === null) {
                $this->_readPreference = $this->db->manager->getReadPreference();
            } elseif (is_scalar($this->_readPreference)) {
                $this->_readPreference = new ReadPreference($this->_readPreference);
            }
        }
        return $this->_readPreference;
    }