Mapping JSON with nested maps to objects

Question

I have a JSON structure that consists of categories. These categories can have different amount of nodes and these nodes have a map of nodes inside of them aswell. The example structure of this JSON looks like this:

[
   {
      "id":"category1",
      "name":"Piłka nożna",
      "nodes":[ 
        {
            "id":"node1",
            "name":"Bayern Monachium",
            "parentId":"category1",
            "nodes": [
                {
                    "id":"node12",
                    "name":"Robert Lewandowski",
                    "parentId":"node1",
                    "nodes": []    
                },
                {
                    "id":"node13",
                    "name":"Thomas Mueller",
                    "parentId":"node1",
                    "nodes": []                
                }
            ]
         },
         {
            "id":"node2",
            "name":"Hertha Berlin",
            "parentId":"category1",
            "nodes": []
         },
         {
            "id":"node5",
            "name":"Werder Brema",
            "parentId":"category1",
            "nodes": []
         }
      ]
   },
   {
      "id":"category2",
      "name":"Koszykówka",
      "nodes": []
   }
]

I have written classes which would allow to represent this JSON with objects:

class Category {
  String id;
  String name;
  Map<String,Node> nodes;
  
  Category(String id, String name, Map<String,Node> nodes) {
    this.id = id;
    this.name = name;
    this.nodes = nodes;
  }
}

class Node {
  String id;
  String name;
  String parentId;
  Map<String, Node> nodes;
  
  Node(String id, String name, String parentId, Map<String, Node> nodes) {
    this.id = id;
    this.name = name;
    this.parentId = parentId;
    this.nodes = nodes;
  }
}

What would be the proper way to map the JSON of this type into the instances of these classes?

Answer 1

An option is to loop over nodes List and create a Node class from each one of them. Here, I implemented a fromJson nammed constructor but dart packages exist to ease the process of deserialization such as json_serializable.

class Node {
  String id;
  String name;
  String parentId;
  List<Node> nodes;
  
  Node.fromJson(Map<String, dynamic> json) {
    this.id = json["id"];
    this.name = json["name"];
    this.parentId = json["parentId"];
    this.nodes = json["nodes"].map<Node>((node) {
      return Node.fromJson(node);
    }).toList();
  }
}

class Category {
  String id;
  String name;
  List<Node> nodes;
  
  Category.fromJson(Map<String, dynamic> json) {
    this.id = json["id"];
    this.name = json["name"];
    this.nodes = json["nodes"].map<Node>((node) {
      return Node.fromJson(node);
    }).toList();
  }
}

  
final categories = [json list].map<Category>((category) => Category.fromJson(category)).toList();