I have never been capable of finding an answer to a predicate that filters on an optionally available one to many relationship. The connection must be optionally available to be able to work with CloudKit. I’ve recreated this with a quite simple challenge. Let’s assume I am on the lookout for a mother and father with a child by the identify of ‘Abbijean’.

Mannequin.swift

@Mannequin
class Father or mother {
    let identifier: UUID
    let identify: String
    @Relationship(deleteRule: .cascade, inverse: Child.guardian) var youngsters: [Kid]?
    init( identify: String, youngsters: [Kid]? = nil) {
        self.identifier = UUID()
        self.identify = identify
    }
}

@Mannequin
class Child {
    let identifier: UUID
    let identify: String
    var guardian: Father or mother?
    init(identify: String, guardian: Father or mother? = nil) {
        self.identifier = UUID()
        self.identify = identify
    }
}

ContentView.swift

struct ContentView: View {
    @Setting(.modelContext) var modelContext
    @Question(filter: #Predicate<Father or mother> { guardian in
//        guardian.youngsters?.accommodates(the place: {
//            $0.identify == "Abbiejean"
//        }) != nil
        guardian.youngsters.flatMap { youngsters in
            youngsters.accommodates(the place: { $0.identify == "Abbijean" })
        } == true

    }) var mother and father: [Parent]
    var physique: some View {
        Button(motion: {
            var parent1 = Father or mother(identify: "Sterling Archer")
            parent1.youngsters?.append(Child(identify: "Abbiejean"))
            modelContext.insert(parent1)
        }, label: {
            Textual content("Add Knowledge")
        })
        Record {
            ForEach(mother and father, id: .id) { guardian in
                Textual content("(guardian.identify)")
            }
        }
    }
}

When utilizing:

guardian.youngsters?.accommodates(the place: {
   $0.identify == "Abbiejean"
}) != nil

The compiler tells me to make use of a flatMap when unwrapping optionals

Utilizing a flat map:

guardian.youngsters.flatMap { youngsters in
    youngsters.accommodates(the place: { $0.identify == "Abbijean" })
} == true

The app crashes with: Thread 1: “to-many key not allowed right here”

Examined in Xcode 15.0.1 (iOS 17.0.1) and 15 Beta 3 (iOS 17.2)