Add a brand-new use case

Goal: add a capability the platform doesn't have — here, export the current events listing as CSV — as a plain new bean. A brand-new use case has no platform seam, so there is nothing to disable: it is just a @Service.

• Seam: none (new capability). Reuses existing seams as collaborators.
• City files: one @Service, plus an endpoint to reach it.
• Applies to: both topologies.

Recipe

A new use case is an ordinary Spring bean. It can compose existing platform seams — here it injects GetEventsUseCase (the platform default, or your override, whichever is active) and reformats its output.

// com.modelcity.leisure.events.usecase.ExportEventsCsvUseCase
@Service
@RequiredArgsConstructor
public class ExportEventsCsvUseCase {

    private final GetEventsUseCase<EventSummaryDto> getEventsUseCase;

    public String execute(EventType type, Boolean paid, String locale) {
        StringBuilder csv = new StringBuilder("id,name,startsAt\n");
        getEventsUseCase.execute(type, paid, 0, locale).forEach(e ->
                csv.append(e.getId()).append(',')
                   .append(e.getName().replace(",", " ")).append(',')
                   .append(e.getStartsAt()).append('\n'));
        return csv.toString();
    }
}

Expose it with a new endpoint on your overriding controller (see Add a new endpoint):

@GetMapping(value = "/export.csv", produces = "text/csv")
public String exportCsv(@RequestParam(required = false) EventType type,
                        @RequestParam(required = false) Boolean paid,
                        Locale locale) {
    return exportEventsCsvUseCase.execute(type, paid, SupportedLocale.from(locale).code());
}

Notes

• No @ModelCityExtensionPoint / @ModelCityDisabledIfInherited — those mark platform seams and their defaults. A city-owned bean under com.modelcity.<vertical> is picked up by the existing component scan directly.
• If the new use case needs its own persistence query, add a repository as in Add a query or filter; if it needs new tables, see Add a new entity.
• You are free to keep the single-method shape (execute) the platform uses for consistency, but ArchUnit's one-method rule only applies to @ModelCityExtensionPoint seams, not your own beans.

Verify

curl "/events/export.csv" returns a CSV of the first page of active events, localised to the Accept-Language header.