How to get values from API with dynamic dates? Dart/Flutter

Question

I have an issue with fetching data from API, my goal is to get separated data like only dates and only values of currency. Basically at some point in trying to solve this problem I've got dates as keys but couldn't get the values of currency. Now I'm getting only nulls.

1. The response is:

{
  "base": "EUR",
  "end_date": "2020-06-06",
  "rates": {
    "2020-06-01": {
      "USD": 1.112558
    },
    "2020-06-02": {
      "USD": 1.118756
    },
    "2020-06-03": {
      "USD": 1.123471
    },
    "2020-06-04": {
      "USD": 1.133657
    },
    "2020-06-05": {
      "USD": 1.129095
    },
    "2020-06-06": {
      "USD": 1.12915
    }
  },
  "start_date": "2020-06-01",
  "success": true,
  "timeseries": true
}

2. This is what I use to retrieve that data. But I don't know how to properly go through the random dates

import 'dart:convert';
import 'package:currency_exchange/models/currency.dart';
import 'package:currency_exchange/models/rate_timeseries.dart';

class CurrencyTimeseries {
  CurrencyTimeseries({
    required this.base,
    required this.endDate,
    required this.rates,
    required this.startDate,
    required this.success,
    required this.timeseries,
  });

  Currency base;
  DateTime endDate;
  Rate rates;
  DateTime startDate;
  bool success;
  bool timeseries;

  factory CurrencyTimeseries.fromJson(
          Map<String, dynamic> json, Currency symbol) =>
      CurrencyTimeseries(
        base: Currency.values
            .firstWhere((element) => element.name == json['base']),
        endDate: DateTime.parse(json["end_date"]),
        rates: Rate.fromJson(json["rates"]),
        success: json["success"] == "true",
        startDate: DateTime.parse(json["start_date"]),
        timeseries: json["timeseries"],
      );
}

3. Here is class Rate used for this code above

import 'dart:convert';

class Rate {
  Rate({
    required this.eur,
  });

  double eur;

  factory Rate.fromJson(Map<String, dynamic> json) => Rate(
        eur: json["EUR"],
      );
}

4. The last one is here, how I send req to API, and its working

Future<CurrencyTimeseries> fetchTimeseries(
    Currency base,
    DateTime endDate,
    DateTime startDate,
    Currency symbol,
  ) async {
    RequestOptions req = RequestOptions(
      path: '/timeseries',
      queryParameters: {
        'base': base.name,
        'end_date': DateFormat('yyyy-MM-dd').format(endDate),
        'start_date': DateFormat('yyyy-MM-dd').format(startDate),
        'symbols': symbol.name,
      },
      baseUrl: _apiUrl,
      headers: {'apikey': _apiKey},
      method: 'GET',
    );

    Response response = await _dio.fetch(req);
    print(response.data);
    return CurrencyTimeseries.fromJson(
      response.data,
      symbol,
    );
  }

Answer 1

This is how to access rates:

final startDate = json["start_date"];
final endDate = json["end_date"];
final startDateUSD = json["rates"][startDate]["USD"];
...
final endDateUSD = json["rates"][endDate]["USD"];