How to wait on a future that returns void? - swift

Question

I am following the book "Server Side Swift Vapor Edition" and I am trying to work on the exercises at page 174.
I have a struct called Poll, defined this way:
struct Poll: Content, SQLiteUUIDModel, Migration {
var id: UUID?
var title: String
var option1: String
var option2: String
var votes1: Int
var votes2: Int
}
It is mapped into a SQLite database with Fluent, and I am trying to write a route that given a post request like this one:
localhost:8080/polls/delete/
Is able to find the poll object in the database, return an error if it doesn't exist, or delete if it exists. This is how I am solving the problem at the moment:
router.post("polls", "delete", UUID.parameter) { req -> Future<Poll> in
let id = try req.parameters.next(UUID.self)
return Poll.find(id, on: req).map(to: Poll.self) { poll in
guard let poll = poll else {
throw Abort(.notFound)
}
poll.delete(on: req)
return poll
}
}
Let's break it down:
I read for the UUID passed in the post request (e.g. http://localhost:8080/polls/delete/FBF7FDC2-0ECB-4C1F-AD8F-A62DE68E531B)
I try to find the poll
If the poll object is nil (which means that it was not found), I throw a 404 not found error
If I find it, I delete the poll and I return it
This works. I am able to delete polls by using this route.
But I have some questions in my mind:
Can it be that the delete method still fails? (e.g. because of an internal SQLite error)
If yes, can I wait until the poll is actually deleted before returning it?
The problem is that the delete method returns an object of type EventLoopFuture. If it was an object of tupe EventLoopFuture, I would be able to easily map it to a poll. But being that the template argument is Void, if I modify the code this way:
router.post("polls", "delete", UUID.parameter) { req -> Future<Poll> in
let id = try req.parameters.next(UUID.self)
return Poll.find(id, on: req).flatMap(to: Poll.self) { poll in
guard let poll = poll else {
throw Abort(.notFound)
}
return poll.delete(on: req).flatMap(to: Poll.self) { poll -> EventLoopFuture<Poll> in
return poll
}
}
}
I get a syntax error: "Cannot convert value of type 'Void' to closure result type 'EventLoopFuture'". It looks like I am not able to map an EventLoopFuture object to EventLoopFuture. The problem is just that I want to wait for the delete operation to complete before returning. Any solution?

Answer 1

The delete function returns Void so your closure shouldn't have a parameter, which is one reason you are getting the syntax error. Try this:
router.post("polls", "delete", UUID.parameter) { req -> Future<Poll> in
let id = try req.parameters.next(UUID.self)
return Poll.find(id, on: req).flatMap { poll in
guard let poll = poll else {
throw Abort(.notFound)
}
return poll.delete(on: req).flatMap{
return request.future(poll)
}
}
}
Taking OP's comments to #Rob Napier's answer on-board, this should convert your poll back to a future but only after the delete has completed.