@Attribute 'unique' and complex keys

I have a similar question also, for now I would create and UUID instead of complex key, e.g.:

@Attribute(.unique) var identifier: UUID

I tested define @Attribute(.unique) multiple times and seems not a correct solution:

@Model final class Book {
    @Attribute(.unique)
    var title: String
    @Attribute(.unique)
    var author: String
}

I tried to insert books have some wried result:

container.mainContext.insert(BooK(title: "book 1", author: "author 1"))
try? container.mainContext.save()

// 1 book inserted

container.mainContext.insert(BooK(title: "book 1", author: "author 1"))
try? container.mainContext.save()

// 1 book inserted as expected

container.mainContext.insert(BooK(title: "book 2", author: "author 1"))
try? container.mainContext.save()

// "book 2" replace "book 1", and 1 book is left.

My workaround: create our own unique key with title and author:

@Model final class Book {
    @Attribute(.unique)
    var titleAndAuthor: String    
    
    var title: String
    var author: String

    init(title: String, author: String) {
        self.title = title
        self.author = author
        titleAndAuthor = self.title + self.author
    }
}

Now you can declare unique constraints as part of the model definition.

Like so:

@Model final class Book {
    #Unique<Person>([\.title], [\.author])    
    
    var title: String
    var author: String
 
    init(title: String, author: String) {
        self.title = title
        self.author = author
    }
}

This is a very nice addition to SwiftData, but there is still a problem (which I didn't mention in the original post). To the best of my knowledge, CloudKit still does not support (or enforce) unique properties.

So, unless I'm missing something, this solution only works if you don't want take advantage of iCloud data synchronization services. Or am I missing something?