Reverse relationships for models are not always set (iOS 18 RC)

Question

Created Sep ’24

Replies 1

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Participants 2

I'm running into an odd case where a model's reverse relationship is sometimes not set despite the forward relationship being there.

If the app is closed and reopened however, the reverse relationship for previously added data works.

For example, given three models Shelf, Item and ItemDetails:

@Model final class Shelf { 
    @Relationship(deleteRule: .cascade, inverse: \Item.primaryShelf)    
    var items: [Item] = []    
    init() {}
}

@Model final class Item {
    var primaryShelf: Shelf?
    var timestamp: Date

    @Relationship(deleteRule: .cascade, inverse: \ItemDetail.item)
    public var detail: ItemDetail?

    init(primaryShelf: Shelf) {
        self.primaryShelf = primaryShelf
        self.timestamp = .now
    }
}

@Model final class ItemDetail {
    var item: Item?        
    init(item: Item) { self.item = item }
}

Now I want to simply create a shelf, some items and some itemdetails.

@Test func testRelationshipsThroughInit() async throws {        
    let schema = Schema([Shelf.self, Item.self, ItemDetail.self])
    let config = ModelConfiguration(schema: schema, isStoredInMemoryOnly: true)        
    let container = try ModelContainer(for: schema, configurations: [config])        
    let modelContext = ModelContext(container)                
    let shelf = Shelf()        
    modelContext.insert(shelf)   
             
    for _ in 0..<10 {            
        let item = Item(primaryShelf: shelf)            
        modelContext.insert(item)            
        let itemDetail = ItemDetail(item: item)
        modelContext.insert(itemDetail)        
    }                
    try modelContext.save()               
 
    let fetchDescriptor = FetchDescriptor<Shelf>()        
    let shelves = try modelContext.fetch(fetchDescriptor)                     

    // fails with a random number between 0 and 9 typically
    #expect(shelves.first?.items.count == 10)       
}

There seem to be two ways that this problem goes away. The first is changing the order of properties set in ItemDetail.init() so that the relationship is set after everything else:

@Model final class Item {
    // ...
    init(primaryShelf: Shelf) {        
        self.timestamp = .now        
        self.primaryShelf = primaryShelf    
    }

With this, everything seems to work fine. The other way seems to be manually setting the reverse relationship. So the loop above gets changed to:

    for _ in 0..<10 {            
        let item = Item(primaryShelf: shelf)
        modelContext.insert(item)
        let itemDetail = ItemDetail(item: item)
        modelContext.insert(itemDetail)

        // add reverse relationship even though forward was set
        shelf.items.append(item)        
    }

My question is, is this the expected behavior and If so, is there any place this is documented?

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Answer 1

DTS Engineer OP

Apple

Sep ’24

Recommended

No, you don't need to set the reverse relation. When you set a relationship, SwiftData should set the reverse for you.

When working with SwiftData relationships, there is a rule of thumb – Inserting the model objects to the model context before related them. This is because when you change a relationship of a model object, SwiftData needs to update the related objects so the relationship stays consistent, and performing the update needs to access the model context.

Concretely in your case, you might try to create your object graph in the following way:

@Model final class Item {
    ...
    //init(primaryShelf: Shelf) {
    init() {
        //self.primaryShelf = primaryShelf
        self.timestamp = .now
    }
}

for _ in 0..<10 {            
	let item = Item(primaryShelf: shelf)            
	modelContext.insert(item)    
	item.primaryShelf = primaryShelf
    ...
}

I believe this is an area that SwiftData can be more intuitive when setting up a relationship, and so would suggest that you file a feedback report – If you do so, please share your report ID here for folks to track.

Best,
——
Ziqiao Chen
 Worldwide Developer Relations.

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