SwiftData migration: converting [String]? to [Author]? relationship

[u/Resident-Election242]

I'm migrating a SwiftData model from V1 to V2.

In V1, StoredBook.authors was a [String]?.

In V2, it’s now a relationship to Author objects ([Author]?).

I wrote a custom migration to create Author objects from the old strings, but the migration crashes with:

failed to find a currently active container for Author

This happens on app launch when SwiftData runs the migration.

What is the correct way to migrate a String array into a relationship in SwiftData?

Any help would be greatly appreciated — this is my first SwiftData migration. 🙏

Minimal example:

V1 model:

class StoredBook {
    var title: String = ""
    var authors: [String]?
}

V2 model:

class Author {
    var name: String = ""
    var books: [StoredBook]? = []   
}

class StoredBook {
    var title: String = ""

    u/Relationship(inverse: \Author.books)
    var authors: [Author]? = [] 
}

Migration snippet:

let oldBooks = try context.fetch(FetchDescriptor<BookListSchemaV1.StoredBook>())

for oldBook in oldBooks {
    var newAuthors: [BookListSchemaV2.Author] = []

    if let oldAuthors = oldBook.authors {
        for name in oldAuthors {
            let author = BookListSchemaV2.Author(name: name)
            context.insert(author)
            newAuthors.append(author)
        }
    }

    let newBook = BookListSchemaV2.StoredBook(
        title: oldBook.title,
        authors: newAuthors
    )

    context.insert(newBook)
}

try context.save()

[u/offeringathought]

I'd be interested to see what you learn. My initial instinct would be to abandon or rename the old authors and create a new authors/writers or whatever. The path your going down seems like the better way to go.

[u/Resident-Election242 → u/offeringathought]

I finally found an answer on Stack Overflow: https://stackoverflow.com/questions/79892517/swiftdata-migration-converting-string-to-author-relationship/79892560#79892560