code: public function getPrimaryKey()
      {
          $pk = $this->meta->schema->primaryKey;
          if (is_array($pk)) {
              $ppk = [];
              foreach ($pk as $key) {
                  if (!isset($this->attributes[$key])) {
                      $ppk[] = null;
                  } else {
                      $ppk[] = $this->attributes[$key];
                  }
              }
  
              return implode('.', $ppk);
          }
          if (!isset($this->attributes[$pk])) {
              return;
          }
  
          return $this->attributes[$pk];
      }
docs: Get primary key.
  
  @return string the primary key