Event ordering when setting pageSize is wrong

[t-persson]

Description

The ordering of events in the event repository is such that the latest event comes last and the oldest event comes first, this means that when we limit the size of the page with pageSize we will only get the oldest event instead of the latest.
It is possible to get the latest event if pageNo is set to totalNumberItems, but that just feels weird.

Motivation

I want to limit my query to the ER so that I only get the latest event.

Exemplification

Limiting the database & ER query in order to play nice with the services by setting pageSize=1 is not really possible.

Benefits

Easier to play nice with the service.

Possible Drawbacks

None

[magnusbaeck]

Is there anything more to this than just sorting allEvents before returning from GetEvents? We could maintain allEvents as a sorted slice at all time rather than just appending the events (line 191) but I don't know if that would actually be more efficient (and if it would matter given the relatively small number of elements).

eiffel-goer/internal/database/drivers/mongodb/mongodb.go

Lines 157 to 201 in d4f7bb3

	func (m *Database) GetEvents(ctx context.Context, request requests.MultipleEventsRequest) ([]drivers.EiffelEvent, int64, error) {
	filter, err := buildFilter(request.Conditions)
	if err != nil {
	m.logger.Errorf("Database: %v", err)
	return nil, 0, err
	}
	collections, err := m.collections(ctx, filter)
	if err != nil {
	m.logger.Errorf("Database: %v", err)
	return nil, 0, err
	}
	m.logger.Debugf("fetching events from %d collections", len(collections))
	allEvents := make([]drivers.EiffelEvent, 0, request.PageSize)
	var numberOfDocuments int64
	for _, collection := range collections {
	var events []drivers.EiffelEvent
	limit := request.PageSize - int32(len(allEvents))
	col := m.database.Collection(collection)
	if limit > 0 {
	cursor, err := col.Find(ctx, filter, options.Find().
	SetProjection(bson.M{"_id": 0}).
	SetSkip(int64((request.PageNo-1)*request.PageSize)).
	SetLimit(int64(limit)),
	)
	if err != nil {
	continue
	}
	if err = cursor.All(ctx, &events); err != nil {
	m.logger.Info(err.Error())
	continue
	}
	allEvents = append(allEvents, events...)
	}
	if len(events) > 0 && int32(len(events)) < limit && request.PageNo == 1 {
	numberOfDocuments += int64(len(events))
	} else {
	count, _ := col.CountDocuments(ctx, filter, &options.CountOptions{})
	numberOfDocuments += count
	}
	}
	return allEvents, numberOfDocuments, nil
	}