Dart - GroupBy with nested lists - list

Question

Could use a Dart hero. Kinda new so let me try to explain short but correctly
I have the following object structure
Client has a List<Project>
Project has a List<Registration>
Registration has a field date which is of type DateTime
class Client {
String id;
String name;
String hexColor;
List<Project> projects;
}
class Project {
String id;
String name;
List<Registration> registrations;
}
class Registration {
String id;
String description;
DateTime date;
int minutesWorked;
}
So currently I have a List<Client> but I want to do a GroupBy based on the Registration.date value... how to do a groupBy with a nested list?
Expected output:
[
2020-01-01 00:00:00.000: [Instance of Client, Instance of Client, Instance of Client],
2020-01-02 00:00:00.000: [Instance of Client]
]
how on earth should this be pulled off!

Answer 1

How about:
Map<DateTime, List<Client>> clientsByRegsitrationDate(Iterable<Client> clients) {
var result = <DateTime, List<Client>>{};
for (var client in clients) {
for (var registration in client.registrations) {
result.putIfAbsent(registration.date, () => []).add(client);
}
}
return result;
}
This assumes that all registration dates for the same client are different, otherwise the same client can end up in a list more than once.
If you want to avoid that, maybe:
Map<DateTime, List<Client>> clientsByRegsitrationDate(Iterable<Client> clients) {
var result = <DateTime, List<Client>>{};
for (var client in clients) {
for (var registration in client.registrations) {
var list = result.putIfAbsent(registration.date, () => []);
if (list.isEmpty || !identical(list.last, client)) list.add(client);
}
}
return result;
}
Or just use sets instead of lists to collect the clients:
Map<DateTime, Set<Client>> clientsByRegsitrationDate(Iterable<Client> clients) {
var result = <DateTime, List<Client>>{};
for (var client in clients) {
for (var registration in client.registrations) {
result.putIfAbsent(registration.date, () => {}).add(client);
}
}
return result;
}
That also avoids double-counting clients if they occur more than once in clients.