Swift Data Migration not working

I am trying to convert a string field to an integer field in our database schema. However, the custom migration that I write doesn't seem to run.

My Model

//Run 1

//typealias Book = BookSchemaV1.Book

//Run 2
typealias Book = BookSchemaV2.Book

// MARK: - Migration Plan
enum BookModelMigrationPlan: SchemaMigrationPlan {
  static var schemas: [any VersionedSchema.Type] = [
    BookSchemaV1.self,
    BookSchemaV2.self
  ]
  
  static var stages: [MigrationStage] = [migrateV1toV2]
  
  static var oldBooks: [BookSchemaV1.Book] = []
  
  static let migrateV1toV2 = MigrationStage.custom(
    fromVersion: BookSchemaV1.self,
    toVersion: BookSchemaV2.self,
    willMigrate: nil,
    didMigrate: { context in
      oldBooks = try context.fetch(FetchDescriptor<BookSchemaV1.Book>())
      oldBooks.forEach { oldBook in
        do {
          let newBook = BookSchemaV2.Book(
            bookID: String(oldBook.bookID),
            title: oldBook.title
          )
          context.insert(newBook)
          context.delete(oldBook)
          try context.save()
        } catch {
          print("New model not saved")
        }
      }
    }
  )
}

// MARK: - Schema Versions
enum BookSchemaV1: VersionedSchema {
  
  static var models: [any PersistentModel.Type] = [Book.self]
  static var versionIdentifier = Schema.Version(1, 0, 0)
  
  @Model
  final class Book {
    @Attribute(.unique) var bookID: Int
    var title: String
    
    init(
      bookID: Int,
      title: String
    ) {
      self.bookID = bookID
      self.title = title
    }
  }
}

enum BookSchemaV2: VersionedSchema {
  
  static var models: [any PersistentModel.Type] = [Book.self]
  static var versionIdentifier = Schema.Version(2, 0, 0)
  
  @Model
  class Book {
    @Attribute(.unique) var bookID: String
    var title: String
    
    init(
      bookID: String,
      title: String
    ) {
      self.bookID = bookID
      self.title = title
    }
  }
}

@MainActor
class AppDataContainer {
  static let shared = AppDataContainer()
  
  let container: ModelContainer
  
  private init() {
    do {
      let schema = Schema([Book.self])
      let config = ModelConfiguration(schema: schema)
      container = try ModelContainer(for: schema, migrationPlan: BookModelMigrationPlan.self, configurations: [config])
    } catch {
      fatalError("Could not create ModelContainer: \(error)")
    }
  }
}