Looping sections with grouped values from model

I am attempting to place drugs under sections correlating to their class but I’m getting multiple sections for each drug I add of the same class. I’m not sure how much this process differs when using model macros.

Current Result

List {
    ForEach(activeStash) { drug in
        Section(header: Text(drug.substanceClass)) {
            NavigationLink(destination: StashLinkView(stashed: drug)) {
                Text(drug.substance)
            }
        }
    }
}

Grouping but with duped sections Result:

List {
    ForEach(activeStash) { drug in
        Section(header: Text(drug.substanceClass)) {
            ForEach(activeStash) { drug in
                NavigationLink(destination: StashLinkView(stashed: drug)) {
                    Text(drug.substance)
                }
            }
        }
    }
}

Solution:

List {
    ForEach(Array(Set(activeStash.map { $0.substanceClass })), id: \.self) { substanceClass in
        Section(header: Text(substanceClass)) {
            ForEach(activeStash.filter { $0.substanceClass == substanceClass }) { drug in
                NavigationLink(destination: StashLinkView(stashed: drug)) {
                    Text(drug.substance)
                }
            }
        }
    }
}

Your first ForEach needs to filter the available substance classes (the sections), the second filters drugs by their substance class to appear in the correct section:

struct Drug: Identifiable, CustomStringConvertible {
    let substance: String
    let substanceClass: SubstanceClass
    
    let id: UUID = UUID()
    
    var description: String {
        "Drug: \(substance) (\(substanceClass))"
    }
}

enum SubstanceClass: String, CaseIterable, Identifiable {
    case stimulant = "Stimulant"
    case sedative = "Sedative"
    
    var id: String {
        self.rawValue
    }
}

struct ContentView: View {
    @State var substances: [Drug] = [
        Drug(substance: "Caffeine", substanceClass: .stimulant),
        Drug(substance: "Adrafinil", substanceClass: .stimulant),
        Drug(substance: "Valerian", substanceClass: .sedative)
    ]
    
    var body: some View {
        VStack {
            Text("Drug Store")
            List {
                ForEach(SubstanceClass.allCases) { substanceClass in
                    Section(header: Text(substanceClass.rawValue)) {
                        ForEach(substances.filter({$0.substanceClass == substanceClass})) { drug in
                            NavigationLink(destination: Detail(drug: drug)) {
                                Text(drug.substance)
                            }
                        }
                    }
                }
            }
        }
        .padding()
    }
}

struct Detail: View {
    let drug: Drug
    
    var body: some View {
        Text(drug.description)
    }
}

I made up the model from what was available in your question.