helper_toJson_fromJson_trick.dart

 Map<String, dynamic> toJson() {
    List<Map>? orderProducts = this.orderProducts != null
        ? this.orderProducts!.map((i) => i.toJson()).toList()
        : null;

    return {
      'pos_id': posId,
      'sector_id': sectorId,
      'seller_id': sellerId,
      "date": date == null ? null : date!.toIso8601String(),
      'motif_no_order': motifNoOrder,
      'order_products': orderProducts
    };
  }

 OrderExtension.fromJson(Map<String, dynamic> json)
      : posId = json['pos_id'],
        sectorId = json['sector_id'],
        sellerId = json['seller_id'],
        date = DateTime.tryParse(json["date"]),
        motifNoOrder = json['motif_no_order'],
        orderProducts = json['order_products']
            .map((orderProduct) => OrderProductExtension.fromJson(orderProduct))
            .toList();