private function _resolveRelations($attr)
    {
        $many = self::_checkAndReturnMany($attr, $this->_data[self::getPK()]);
        if ($many) {
            return $many;
        }

        $belongs = $this->_checkAndReturnBelongs($attr, $this->_data);
        if ($belongs) {
            return $belongs;
        }

        $has_one = $this->_checkAndReturnHasOne($attr, $this->_data[self::getPK()]);
        if ($has_one) {
            return $has_one;
        }
        return null;
    }