Flutter converting Nested Object from Json returns null

Question

I have a Nested Object like this just a bit bigger:

"name": "String",       
    "exercise": [               
                   {                
                   "index": 1,                                  
                   }            
            ],              
    "pause": [              
        {"index":2},                        
    ]           

I convert the exercise and pause to a Json String and save them in a column in SQFLite.

The problem

When I read the Data everything works fine including the List (not nested) but both list's of nested Object are empty when I read a value of the nested object it gives an error.

item.exercise[0].index.toString()

Valid Value range is empty: 0

When I read only item.exercise.toString() it returns []. Without != null ? [...] : List<Exercise>() it also throws an error

Data I get from my Database (shortened)

List of:

[{name: number 1, id: 56, exercise: [{"index":1,"weightGoal":[15,16,17]}, {"index":3,"weightGoal":[15,16,17]}], pause: [{"index":2}]},{"index":4}]}]

What I do with it

Here I try to go through the list and convert it into a List of PlanModel:

List<PlanModel> list =
        res.isNotEmpty ? res.map((c) => PlanModel.fromJson(c)).toList() : [];
    return list;

Full model

PlanModel planModelFromJson(String str) => PlanModel.fromJson(json.decode(str));

String planModelToJson(PlanModel data) => json.encode(data.toJson());

class PlanModel {
  PlanModel({
    this.name,
    this.id,
    this.workoutDays,
    this.pastId,
    this.timesDone,
    this.exercise,
    this.pause,
  });

  String name;
  int id;
  List<String> workoutDays;
  int pastId;
  int timesDone;
  List<Exercise> exercise;
  List<Pause> pause;

  factory PlanModel.fromJson(Map<String, dynamic> json) => PlanModel(
    name: json["name"],
    id: json["id"],
    workoutDays: List<String>.from(jsonDecode(json["workoutDays"])),
    pastId: json["pastId"],
    timesDone: json["timesDone"],
    exercise: json["Exercise"] != null ? new List<Exercise>.from(json["Exercise"].map((x) => Exercise.fromJson(x))): List<Exercise>(),
    pause: json["Pause"] != null ? new List<Pause>.from(json["Pause"].map((x) => Pause.fromJson(x))): List<Pause>(),
  );

  Map<String, dynamic> toJson() => {
    "name": name,
    "id": id,
    "workoutDays": List<dynamic>.from(workoutDays.map((x) => x)),
    "pastId": pastId,
    "timesDone": timesDone,
    "Exercise": List<dynamic>.from(exercise.map((x) => x.toJson())),
    "Pause": List<dynamic>.from(pause.map((x) => x.toJson())),
  };



}

class Exercise {
  Exercise({
    this.index,
    this.name,
    this.goal,
    this.repGoal,
    this.weightGoal,
    this.timeGoal,
    this.setGoal,
  });

  int index;
  String name;
  int goal;
  int repGoal;
  List<int> weightGoal;
  int timeGoal;
  List<String> setGoal;

  Exercise.fromJson(dynamic json) {
    // anything that is wrapped around with this [] in json is converted as list
    // anything that is wrapped around with this {} is map
    index = json["index"];
    name = json["name"];
    goal = json["goal"];
    repGoal = json["repGoal"];
    weightGoal = json["weightGoal"] != null ? json["weightGoal"].cast<int>() : [];
    timeGoal = json["timeGoal"];
    setGoal = json["setGoal"] != null ? json["setGoal"].cast<String>() : [];
  }

  Map<String, dynamic> toJson() => {
    "index": index,
    "name": name,
    "goal": goal,
    "repGoal": repGoal,
    "weightGoal": List<dynamic>.from(weightGoal.map((x) => x)),
    "timeGoal": timeGoal,
    "setGoal": List<dynamic>.from(setGoal.map((x) => x)),
  };
}

class Pause {
  Pause({
    this.index,
    this.timeInMilSec,
  });

  int index;
  int timeInMilSec;

  factory Pause.fromJson(Map<String, dynamic> json) => Pause(
    index: json["index"],
    timeInMilSec: json["timeInMilSec"],
  );

  Map<String, dynamic> toJson() => {
    "index": index,
    "timeInMilSec": timeInMilSec,
  };
}

Answer 1

Read this first.

You need to tweek this code a little to work for you but the idea is that; also read comment in the code.

if json string comes with [] those around, json.decode will decode it as List<Map>.

if it comes with {} this json.decode will decode it as Map.

note: be careful while using generics on json.decode I reccommend not to.

data inside the jsonString does not really corresponds with the values inside the fromJson function. json string which you have provided was not really good. so I think you will understand how to manipulate it for your needs.

also main constructor Exercise you can use for initial data.

import 'dart:convert';
class Exercise{
  Exercise({this.index, 
            this.name, 
            this.repGoal, 
            this.weightGoal, 
            this.setGoal});
  
  String index;
  String name;
  String repGoal;
  String weightGoal;
  String setGoal;


Exercise.fromJson(dynamic json) : 
    // anything that is wrapped around with this [] in json is converted as list
    // anything that is wrapped around with this {} is map
    index = json["exercise"][0]["index"].toString(),
    name = json["name"].toString(),
    repGoal = json["repGoal"].toString(),
    weightGoal = json["weightGoal"].toString(),
    setGoal = json["setGoal"].toString();
  
  
}

void main(){
  String jsonString = '{name: number 1, id: 56, exercise: [{"index":1,"weightGoal":[15,16,17], pause: [{"index":2}]}';
  Map json = json.decode(jsonString);
  Exercise.fromJson(json);
  
}