public function getRelationTable()
    {
        if (is_null($this->_relationTable)) {
            $names = [
                $this->getEntity()->getTableName(),
                Entity\Manager::getInstance()
                    ->get($this->_relatedEntity)
                    ->getEntity()->getTableName()
            ];
            asort($names);
            $this->setRelationTable(implode('_', $names));
        }
        return $this->_relationTable;
    }