code: public function primaryKey()
      {
          $pk = $this->meta->schema->primaryKey;
          if (is_array($pk)) {
              $ppk = [];
              foreach ($pk as $key) {
                  $ppk[] = $key;
              }
  
              return implode('.', $ppk);
          }
  
          return $pk;
      }
docs: Retrieve the primary key column for the model item.
  
  @return string the primary key