How to decode JSON in Flutter when the keys are the values

Question

I'm trying to decode the following JSON in Flutter and I am only getting the first key/value pair in my response. The first level keys (i.e. "Product Line 1", etc.) will all be dynamic and I need to capture those keys as values to add as a variable in my ProductCodeEntity class. Can someone point out what I'm doing wrong?

[
  {
    "Product line 1": [
      {"partNumber": "160-9013-900", "orderable": true, "description": "Part Number 1 Description"},
      {"partNumber": "160-9104-900", "orderable": true, "description": "Part Number 2 Description"},
      {"partNumber": "160-9105-900", "orderable": false, "description": "Part Number 3 Description"}
    ],
    "Product line 2": [
      {"partNumber": "160-9113-900", "orderable": true, "description": "Part Number 4 Description"},
      {"partNumber": "160-9114-900", "orderable": true, "description": "Part Number 5 Description"},
      {"partNumber": "160-9115-900", "orderable": false, "description": "Part Number 6 Description"}
    ],
    "Product line 3": [
      {"partNumber": "160-9205-900", "orderable": true, "description": "Part Number 7 Description"},
      {"partNumber": "160-9211-900", "orderable": true, "description": "Part Number 8 Description"},
      {"partNumber": "160-9212-900", "orderable": false, "description": "Part Number 9 Description"}
    ]
  }
]

My ProductLineEntity class looks like this. I believe the problem is in the factory ProductLineEntity.fromJson() function, but I haven't been able to get it to work.

class ProductLineEntity {
  int? id;
  String? name;
  List<ProductCodeEntity>? productCodeList;

  ProductLineEntity({
    this.id,
    this.name,
    this.productCodeList,
  });

  factory ProductLineEntity.fromJson(Map<String, dynamic> json) {
    final List<ProductCodeEntity> productCodes = (json[json.keys.first] as List)
        .map((productCodeJson) => ProductCodeEntity.fromJson(productCodeJson))
        .toList();

    return ProductLineEntity(
      name: json.keys.first,
      productCodeList: productCodes,
    );
  }
}

My ProductCodeEntity class looks like this:

class ProductCodeEntity {
  int? id;
  String? partNumber;
  bool? orderable;
  String? description;
  String? productLineName;

  ProductCodeEntity({
    this.id,
    this.partNumber,
    this.orderable,
    this.description,
    this.productLineName,
  });

  factory ProductCodeEntity.fromJson(Map<String, dynamic> json) {
    return ProductCodeEntity(
      id: (json['id'] as num?)?.toInt(),
      partNumber: json['partNumber'] as String?,
      orderable: json['orderable'] as bool?,
      description: json['description'] as String?,
      productLineName: json['productLineName'] as String?,
    );
  }
}

Here is my code to read the JSON file:

Future<List<ProductLineEntity>> readJson() async {
  List<ProductLineEntity> productLineFromJson(dynamic str) =>
    List<ProductLineEntity>.from(
      (str as List<dynamic>).map((x) => ProductLineEntity.fromJson(x)));

  try {
    final String jsonData =
      await rootBundle.loadString('assets/part_numbers2.json');

    final response = (json.decode(jsonData) as List<dynamic>);

    List<ProductLineEntity> productLines = productLineFromJson(response);

    return productLines;
  } catch (error) {
    throw Exception(
      'ProductLineEntity, Unexpected error reading JSON: $error');
  }
}

Answer 1

1. JSON structure mismatch: The original JSON data structure is a list containing a single object, while ProductLineEntity.fromJson requires a Map<String, dynamic>, resulting in a type mismatch error.
2. Product line name handling: In the ProductLineEntity.fromJson method, you use json.keys.first to get the key, which results in only the first one being parsed when parsing a JSON object with multiple keys.

Here is the revised code

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:logger/logger.dart';

void main() {
  runApp(const MaterialApp(home: TestPage()));
}

class TestPage extends StatefulWidget {
  const TestPage({super.key});

  @override
  State<TestPage> createState() => _TestPageState();
}

class _TestPageState extends State<TestPage> {
  var logger = Logger(
    printer: PrettyPrinter(),
  );

  @override
  void initState() {
    super.initState();
    logger.init;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: TextButton(
        onPressed: () async {
          List<ProductLineEntity> productLines = await readJson1();
          logger.i(json.encode(productLines));
        },
        child: Text('Read JSON'),
      ),
    );
  }

  Future<List<ProductLineEntity>> readJson1() async {
    List<ProductLineEntity> products = [];

    try {
      final String jsonData =
      await rootBundle.loadString('assets/part_numbers2.json');
      final List<dynamic> response = json.decode(jsonData);

      for (var productLine in response) {
        productLine.forEach((key, value) {
          products.add(ProductLineEntity.fromJson(key, value));
        });
      }

      logger.i(products);

      return products;
    } catch (error) {
      throw Exception(
          'ProductLineEntity, Unexpected error reading JSON: $error');
    }
  }
}

class ProductLineEntity {
  int? id;
  String? name;
  List<ProductCodeEntity>? productCodeList;

  ProductLineEntity({
    this.id,
    this.name,
    this.productCodeList,
  });

  factory ProductLineEntity.fromJson(String key, List<dynamic> value) {
    return ProductLineEntity(
      name: key,
      productCodeList: value.map((e) => ProductCodeEntity.fromJson(e)).toList(),
    );
  }

  Map toJson() {
    Map<String, dynamic> data = {};

    data['name'] = name;
    data['productCodeList'] =
        productCodeList?.map((e) => e.toJson()).toList() ?? [];
    data['id'] = id;

    return data;
  }
}

class ProductCodeEntity {
  int? id;
  String? partNumber;
  bool? orderable;
  String? description;
  String? productLineName;

  ProductCodeEntity({
    this.id,
    this.partNumber,
    this.orderable,
    this.description,
    this.productLineName,
  });

  factory ProductCodeEntity.fromJson(Map<String, dynamic> json) {
    return ProductCodeEntity(
      id: (json['id'] as num?)?.toInt(),
      partNumber: json['partNumber'] as String?,
      orderable: json['orderable'] as bool?,
      description: json['description'] as String?,
      productLineName: json['productLineName'] as String?,
    );
  }

  Map toJson() => {
    'id': id,
    'partNumber': partNumber,
    'orderable': orderable,
    'description': description,
    'productLineName': productLineName
  };
}