code: public function scopeWhereSKU($query, $sku) {
          return $query->join(
                  config('shop.item_table'), 
                  config('shop.item_table') . '.order_id', 
                  '=', 
                  $this->table . '.id'
              )
              ->where(config('shop.item_table') . '.sku', $sku);
      }
docs: Scopes class by item sku.
  Optionally, scopes by status.
  
  @param \Illuminate\Database\Eloquent\Builder $query  Query.
  @param mixed                                 $sku    Item SKU.
  
  @return this