Core Data basics – Part Three

If you’re just stumbling across this, perhaps have a look at Part 1 where I layout a simple master/detail app with the data held as arrays of structs, and Part 2 where I convert that into the simplest possible Core Data version. In this post, I’m going to add the mechanics for the one:many relationship – Each Garden can be associated with multiple Plants.

I should also mention I figured out some of this with help from this video from Jonathan Rasmusson.

First step is another Paul hack. If you look in the Garden+CoreDataProperties.swift file where the Garden properties are defined, you’ll see that the plants variable has the type ?NSSet which is not straightforward to work with. We’d prefer it to be an array of Plant so we can easily turn it into a list in SwiftUI ways.

extension Garden {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Garden> {
        return NSFetchRequest<Garden>(entityName: "Garden")
    }

    @NSManaged public var address: String?
    @NSManaged public var id: UUID?
    @NSManaged public var name: String?
    @NSManaged public var plant: NSSet?
    
    var wrappedName: String {
        name ?? "Unknown"
    }
    var wrappedAddress: String {
        address ?? "Unknown"
    }
    
    var plantArray: [Plant] {
        let set = plant as? Set<Plant> ?? []
        return set.sorted {
            $0.wrappedName < $1.wrappedName
        }
    }

}

Using a computed variable (in this case plantArray), Paul likes to turn it into a Set of Plants, then convert that into an array by sorting it. We have to provide the predicate, but it’s a small price to pay to get the array back. I have concerns about this for a 100K array, but it’s no problem for this app.

Back in our Garden DetailView, it’s now a simple matter to show the plants for a garden in a list under the other garden details.

struct DetailView: View {
    var garden: Garden
    
    var body: some View {
        VStack {
            Text(garden.wrappedName)
                .font(.headline)
            Text(garden.wrappedAddress)
            List {
                ForEach(garden.plantArray, id: \.self) {plant in
                    Text(plant.wrappedName)
                }
            }
        }
    }
}

Finally, in the button press to create our sample data, we need to create each of the plant instances then call a method on a garden to add them to that garden.

Button("Sample Data") {
    let garden1 = Garden(context: moc)
    garden1.id = UUID()
    garden1.name = "White Lodge"
    garden1.address = "72 Anderson Street \nRothwell QLD 4022"
    
    let plant1 = Plant(context: moc)
    plant1.name = "Rose"
    garden1.addToPlant(plant1)
    let plant2 = Plant(context: moc)
    plant2.name = "Palm Tree"
    plant2.garden? = garden1
    garden1.addToPlant(plant2)
    let plant3 = Plant(context: moc)
    plant3.name = "Lawn"
    plant3.garden? = garden1
    garden1.addToPlant(plant3)

The .addToPlant() accessor method is a Swift wrapper for some internal Objective C that was generated for us in the same file as the Garden properties. The bad name ‘AddToPlant’ rather than ‘AddToPlants’ is my bad. When I defined the relationship in Garden (in the data model) I should have called it ‘plants’.

Part 1, Part 2