Cache fetch fallbacks

[Mordil]

extension KeyedCache {
    func fetch<T: Codable>(key: String, on worker: Worker, fallback: @escaping () -> Future<T>) -> Future<T> {
        return get(key, as: T.self)
            .flatMap { cached in
                if let cached = cached {
                    return worker.future(cached)
                }

                return fallback()
                    .flatMap { self.set(key, to: $0).transform(to: $0) }
            }
    }
}

Usage

router.get("cached") { req -> Future<[Cat]> in
    let cache: KeyedCache = try req.make()

    return cache.fetch(key: "cats", on: req) {
        Cat.query(on: req).all() // <-- only executed once
    }
}

router.get("cachedOne") { req -> Future<Cat> in
    let cache: KeyedCache = try req.make()

    guard let name: String = req.query["name"] else {
        throw Abort(.badRequest)
    }

    return cache.fetch(key: "cat-\(name)", on: req) {
        Cat.query(on: req)
            .filter(\Cat.name == name)
            .first()
            .unwrap(or: Abort(.notFound))
    }
}