Flutter/Dart Parsing JSON to model with a Map attribute

You can also try it on https://app.quicktype.io/. This site easily converts your JSON to the dart model class. there are many different languages supported

import 'dart:convert';

SiteHierarchyModel siteHierarchyModelFromJson(String str) => SiteHierarchyModel.fromJson(json.decode(str));

String siteHierarchyModelToJson(SiteHierarchyModel data) => json.encode(data.toJson());

class SiteHierarchyModel {
    SiteHierarchyModel({
        this.workspaces,
        this.zones,
    });

    List<Workspace> workspaces;
    Map<String, List<Workspace>> zones;

    factory SiteHierarchyModel.fromJson(Map<String, dynamic> json) => SiteHierarchyModel(
        workspaces: List<Workspace>.from(json["workspaces"].map((x) => Workspace.fromJson(x))),
        zones: Map.from(json["zones"]).map((k, v) => MapEntry<String, List<Workspace>>(k, List<Workspace>.from(v.map((x) => Workspace.fromJson(x))))),
    );

    Map<String, dynamic> toJson() => {
        "workspaces": List<dynamic>.from(workspaces.map((x) => x.toJson())),
        "zones": Map.from(zones).map((k, v) => MapEntry<String, dynamic>(k, List<dynamic>.from(v.map((x) => x.toJson())))),
    };
}

class Workspace {
    Workspace({
        this.id,
        this.createdAt,
        this.updatedAt,
        this.siteId,
        this.orgId,
        this.name,
        this.workspaceId,
    });

    String id;
    DateTime createdAt;
    DateTime updatedAt;
    String siteId;
    String orgId;
    String name;
    String workspaceId;

    factory Workspace.fromJson(Map<String, dynamic> json) => Workspace(
        id: json["id"],
        createdAt: DateTime.parse(json["createdAt"]),
        updatedAt: DateTime.parse(json["updatedAt"]),
        siteId: json["siteId"],
        orgId: json["orgId"],
        name: json["name"],
        workspaceId: json["workspaceId"] == null ? null : json["workspaceId"],
    );

    Map<String, dynamic> toJson() => {
        "id": id,
        "createdAt": createdAt.toIso8601String(),
        "updatedAt": updatedAt.toIso8601String(),
        "siteId": siteId,
        "orgId": orgId,
        "name": name,
        "workspaceId": workspaceId == null ? null : workspaceId,
    };
}