Core Data: How can one get updates to entities of a relationship?

Question

I know that you can observe changes to entities you've fetched with @ObservedObject.

The problem is best illustrated with an example: If I had an entity school with a one to many relationship to students and each student has a property numberOfPencils then I want to recalculate the sum of all pencils within a school whenever a student in the school changes his numberOfPencils property.

If I access all the students through the schools entity (which is an @ObservedObject) then I only get updates to the sum if a new student is added but not when an existing student changes his numberOfPencils because student objects can only be observed one by one as far as I know.

The only thing I could come up with is setting up a NSFetchedResultsControllerDelegate with a fetchRequestPredicate that gets all the students associated with a school through some keyPath from student to school which uniquely identifies the school. But that seems like a hacky way to do it and there surely must be a better solution that is eluding me.

Basically I want to get notified of changes to any of the students associated with a school.

Does this answer your question? [How would I map over one property of all CoreData entries?](https://stackoverflow.com/questions/66726835/how-would-i-map-over-one-property-of-all-coredata-entries) — lorem ipsum, Jan 15 '23 at 13:59

score 1 · Accepted Answer · answered Jan 15 '23 at 15:27

Derived properties is definely a good way to go.

But to clarify: @ObservedObject can also get updated on change of the item. You just have to manually trigger the update with objectWillChange.send()

Say your School cell looks like this:

struct SchoolCell: View {
    
    @ObservedObject var school: School

    var body: some View {
        HStack {
            let pencils = school.pupilsArray.reduce(0, {$0 + $1.numberOfPencils})
            Text(school.name)
            Spacer()
            Text(verbatim: "\(pencils) pencils")
                .foregroundColor(.secondary)
        }
    }
}

then the Pupil modify would work like this:

// ...
    @ObservedObject var school: School
// ...
    Button("+10 Pencils") {
        school.objectWillChange.send()
        pupil.numberOfPencils += 10
        try? viewContext.save()
    }

It is important though that SchoolCell IS an own struct, so it gets refreshed when its @ObservedObject school changes.

Core Data: How can one get updates to entities of a relationship?

1 Answers1