How to use with a derived request?

[mickeyl]

Consider two model objects, a vehicle:

extension Vehicle: FetchableRecord, PersistableRecord {
    static let scans = hasMany(VehicleScan.self)
    var scans: QueryInterfaceRequest<VehicleScan> {
        request(for: Vehicle.scans)
    }
}

and a scan:

extension VehicleScan: FetchableRecord, PersistableRecord {
    static let vehicle = belongsTo(Vehicle.self)
}

Following your example, creating a @Query like AllVehicles worked fine:

struct AllVehicles: Queryable {
    static var defaultValue: [Vehicle] { [] }

    func publisher(in dbQueue: DatabaseQueue) -> AnyPublisher<[Vehicle], Error> {
        ValueObservation
            .tracking(Vehicle.fetchAll)
            .publisher(in: dbQueue, scheduling: .immediate)
            .eraseToAnyPublisher()
    }
}

I'm not quite sure though how to construct a Queryable that depends on a selected vehicle. I tried that:

struct AllScansForVehicle: Queryable {

    let vehicle: Vehicle

    init(_ vehicle: Vehicle) {
        self.vehicle = vehicle
    }

    static var defaultValue: [VehicleScan] { [] }

    func publisher(in dbQueue: DatabaseQueue) -> AnyPublisher<[VehicleScan], Error> {
        ValueObservation
            .tracking(self.vehicle.scans.fetchAll)
            .publisher(in: dbQueue, scheduling: .immediate)
            .eraseToAnyPublisher()
    }
}

But — of course – I can't instantiate that in SwiftUI:

struct VehicleDetailsView: View {

    @ObservedObject var vehicle: Vehicle
    @Query(AllScansForVehicle(vehicle), in: \.dbQueue) var scans: [VehicleScan]
^^^^Cannot use instance member 'vehicle' within property initializer; property initializers run before 'self' is available
    …
}

How could I achieve that?

[groue]

Hello @mickeyl,

I know a technique, shown in one of the GRDB demo apps: https://github.com/groue/GRDB.swift/blob/v5.22.0/Documentation/DemoApps/GRDBCombineDemo/GRDBCombineDemo/Views/AppView.swift#L18

In your app, this should give:

struct VehicleDetailsView: View {
    @ObservedObject var vehicle: Vehicle
    @Query<AllScansForVehicle> var scans: [VehicleScan]

    init(vehicle: Vehicle) {
        self.vehicle = vehicle
        _scans = Query(AllScansForVehicle(vehicle), in: \.dbQueue)
    }
}

This technique can be extended to all other property wrappers that need a custom initialization!