Group by field, sort and get the first (or last, whatever) items of the group in MongoDB (with Spring Data) - spring

Question

I have the following entity (getters, setters and constructor omitted)
public class Event {
#Id
private String id;
private String internalUuid;
private EventType eventType;
}
EventType is an enum containing arbitrary event types:
public enum EventType {
ACCEPTED,
PROCESSED,
DELIVERED;
}
My problem is that I have a table with a lot of events, some having the same internalUuid but different statuses. I need to get a list of Events with each Event representing the 'newest' status (ordering by EventType would suffice). Currently, I'm just fetching everything, grouping to separates lists in code, sorting the lists by EventType and then just creating a new list with the first element of each list.
Example would be as follows.
Data in table:
{ "id": "1", "internalUuid": "1", "eventType": "ACCEPTED" },
{ "id": "2", "internalUuid": "1", "eventType": "PROCESSED" },
{ "id": "3", "internalUuid": "1", "eventType": "DELIVERED" },
{ "id": "4", "internalUuid": "2", "eventType": "ACCEPTED" },
{ "id": "5", "internalUuid": "2", "eventType": "PROCESSED" },
{ "id": "6", "internalUuid": "3", "eventType": "ACCEPTED" }
Output of the query (any order would be ok):
[
{ "id": "3", "internalUuid": "1", "eventType": "DELIVERED" },
{ "id": "5", "internalUuid": "2", "eventType": "PROCESSED" },
{ "id": "6", "internalUuid": "3", "eventType": "ACCEPTED" }
]
It is not guaranteed that a "higher" status also has a "higher" ID.
How do I do that without doing the whole process by hand? I literally have no idea how to start as I'm very new to MongoDB but haven't found anything that helped me on Google. I'm using Spring Boot and Spring Data.
Thanks!

Answer 1

Okay I think I have figured it out (thanks to Joe's comment). I'm not a 100% sure that the code is correct but it seems to do what I want. I'm open to improvements.
(I had to add a priority field to Event and EventType because sorting by eventType obviously does String-based (alphabetic) sorting on the enum's name):
private List<Event> findCandidates() {
// First, 'match' so that all documents are found
final MatchOperation getAll = Aggregation.match(new Criteria("_id").ne(null));
// Then sort by priority
final SortOperation sort = Aggregation.sort(Sort.by(Sort.Direction.DESC, "priority"));
// After that, group by internalUuid and make sure to also push the full event to not lose it for the next step
final GroupOperation groupByUuid = Aggregation.group("internalUuid").push("$$ROOT").as("events");
// Get the first element of each sorted and grouped list (I'm not fully sure what the 'internalUuid' parameter does here and if I could change that)
final ProjectionOperation getFirst = Aggregation.project("internalUuid").and("events").arrayElementAt(0).as("event");
// We're nearly done! Only thing left to do is to map to our Event to have a usable List of Event in .getMappedResults()
final ProjectionOperation map = Aggregation.project("internalUuid")
.and("event._id").as("_id")
.and("event.internalUuid").as("internalUuid")
.and("event.eventType").as("eventType")
.and("event.priority").as("priority");
final Aggregation aggregation = Aggregation.newAggregation(getAll, sort, groupByUuid, getFirst, map);
final AggregationResults<InvoiceEvent> aggregationResults =
mongoTemplate.aggregateAndReturn(InvoiceEvent.class).by(aggregation).all();
return aggregationResults.getMappedResults();
}