code: public function getParentOf($metaModel)
      {
          $metaModelName = $this->getMetaModelName($metaModel);
  
          return isset($this->childMap[$metaModelName]) ? $this->childMap[$metaModelName] : null;
      }
docs: Get the name of the parenting MetaModel.
  
  @param string $metaModel The name of a child.
  
  @return string