Example of using relationships in a SwiftData preview and passing an instance as an argument.

preview.swift

import SwiftUI
import SwiftData

struct ItemView: View {
    var parent: Parent
    var body: some View {
        VStack {
            Text(parent.id)
            List(parent.children) { child in
                Text("Child \"\(child.id)\" of parent \"\(child.parent!.id)\"")
            }
        }
    }
}

#Preview {
    // create a container
    let schema = Schema([Parent.self, Child.self])
    let config = ModelConfiguration(isStoredInMemoryOnly: true)
    let container = try! ModelContainer(for: schema, configurations: config)
    
    // add data
    let parent = Parent(id: "sup")
    container.mainContext.insert(parent) // always insert the parent before associating a child
    let child = Child(id: "yooooo")
    parent.children.append(child)

    // save.  Not necessary but it helps sometimes
    try! container.mainContext.save()
    
    return ItemView(parent: parent)
        .modelContainer(container)  // don't need a model container in this example but you can pass one anyway
}

@Model
final class Parent {
    @Attribute(.unique) var id: String
    public var children: [Child] = []
        
    init(id: String) {
        self.id = id
    }
}

@Model
final class Child {
    @Attribute(.unique) var id: String
    var parent: Parent? // parent must be optional or swiftdata won't work.  Seems like a bug.

    init(id: String, parent: Parent? = .none) {
        self.id = id
        self.parent = parent
    }
}