code: public function find($id)
      {
          $child = $this->childClassName;
          $model = $child::findWithParentId($id, $this->parent->getId());
          if ($model) {
              $model->setParent($this->parent);
          }
          return $model;
      }
docs: Find model by id.
  
  @param mixed $id
  @return ElasticsearchModel|null