Access list property in Realm migration

[EmDee]

Goals

I've added a primary key to an object that didn't have one before, which is based on some of the object's property, one of which has a List<String> data type. However, I am not able to correctly access the list property, hence I can't write the migration for the new primary key.

Expected Results

Being able to parse the elements in the List property.

Actual Results

Not being able to parse the list.

Code Sample

My database object looks as follows:

@objcMembers
public class TestObject: Object {
    dynamic private(set) var id: String = ""

    let _foobar: List<String> = .init()

    private(set) public var foobar: [SomeEnum] {
        set {
            _foobar.removeAll()
            _foobar.append(objectsIn: newValue.map { $0.rawValue })
        }

        get {
            return _foobar.compactMap { SomeEnum(rawValue: $0) }
        }
    }

    public override class func primaryKey() -> String? {
        return "id"
    }

    convenience init(foo: [SomeEnum]) {
        self.init()

        let foobar = foo.reduce("", { $0 + "-" + $1.rawValue })

        self.id = foobar
        self.foobar = foo
    }
}

In my migration block I'm trying to access _foobar, but can't seem to get it right:

static func migratePrimaryKey(_ migration: RealmSwift.Migration) {
    migration.enumerateObjects(ofType: TestObject.className()) { old, _ in
        let field: String = "_foobar"

        let test1 = old?[field] as? List<String>
        let test2 = old?[field] as? [String]
        let test3 = old?[field] as? List<DynamicObject>
        let test4 = old?[field]
    }
}

Both test and test2 are nil. test3 throws some exceptions:

Could not cast value of type 'NSTaggedPointerString' (0x7fff87a8a270) to 'RealmSwift.DynamicObject' (0x10f065fe8).

The output of test4 shows the following:

(lldb) po test4
▿ Optional<Any>
  - some : List<string> <0x600001fb5f80> (
    [0] someContent
)

There seems to be some technical restriction which prevents me from accessing the actual list properties of my migration object?

Version of Realm and Tooling

Realm framework version:

Realm Object Server version: 3.19.0

Xcode version: 11.2.

iOS/OSX version: 13.2

Dependency manager + version: Carthage 0.34.0

[EmDee]

If my property doesn't contain String, but a custom object instead, I know I can do something along the lines of:

@objcMembers
public class CustomObject: Object {
    dynamic var foo: Int = 0
}

migration.enumerateObjects(ofType: TestObject.className()) { old, new in
    guard let old = old, let new = new else { return }

    let customObjects = old.dynamicList("_foobar")

    for obj in customObjects {
        if let foo = obj["foo"] as? Int {
            print(foo)
        }
    }
}

Are primitive data type handled differently in Realm? If so a workaround would be to have a object wrapping the String, but that seems rather wrong.

[tgoyne]

unsafeBitCast(old![field] as! List<DynamicObject>, to: List<String>.self) will give you a List<String> from the migration object's field. Making this more reasonable is a breaking change for anything currently using that, so I guess it'll have to go into 5.0.