SwiftData Inverse Relationships

SwiftData Inverse Relationships.swift

import SwiftUI
import SwiftData

@Model final class Address {
  let street  = "Am Geldspeicher 1"
  var contact : Contact?
  init() {}
}

@Model final class Contact {
  let name      = "JSON Bourne"
  var addresses : [ Address ] = []
  init() {}
}

struct ContentView: View {
  @Environment(\.modelContext) private var modelContext
  
  @State private var address = Address()
  @State private var contact = Contact()
  
  @State private var reloader = 0 // helper to force refresh all views
  
  struct ShowAddress: View {    
    let address: Address
    let reloader : Int

    var body: some View {
      VStack {
        Text(verbatim: "Address: \(address.persistentModelID.id)")
        Text(address.street)
        if let contact = address.contact { Text("Contact: \(contact.name)") }
        else { Text("No Contact") }
      }
    }
  }
  
  struct ShowContact: View {    
    let contact: Contact
    let reloader : Int

    var body: some View {
      VStack {
        Text(verbatim: "Contact: \(contact.persistentModelID.id)")
        Text(contact.name)
        Text(verbatim: "Addresses: \(contact.addresses)")
      }
    }
  }

  var body: some View {
    VStack {
      ShowAddress(address: address, reloader: reloader)
      Divider()
      ShowContact(contact: contact, reloader: reloader)
      Divider()
      
      // Those properly add the inverse! But don't trigger Observation.
      Button("Add Address to Contact") { contact.addresses.append(address) }
      Button("Add Contact to Address") { address.contact = contact         }

      // Trigger a refresh of all views, makes the updates visible
      Button("Reload") { reloader += 1 }
    }
    .onAppear {
      modelContext.insert(address)
      modelContext.insert(contact)
    }
  }
}

@main struct TestApp: App { 
  let container = try! ModelContainer(
    for: Contact.self, Address.self,
    configurations: .init(isStoredInMemoryOnly: true)
  )
  
  var body: some Scene {
    WindowGroup { ContentView() }
      .modelContainer(container)
  }
}

On startup, no relationships are setup:

After pressing "Add address to contact", the contact view refreshes, but the address one doesn't, despite that the address view observes the contact relationship (and the contact inverse is properly set)

Pressing "refresh" makes the (already established) change show up on the inverse as well.

It was pointed out that explicitly setting the relationship on both sides fixes the issue, e.g.:

      Button("Add Address to Contact") {
        contact.addresses.append(address)
        address.contact = contact
      }

That makes it work. Because the explicit setter is tracked by Observation. (the relationship is set already!)