我正在进行“中等重量”核心数据迁移。我使用映射模型从一个旧存储/数据模型迁移到不同的存储和不同的模型(即完全不同的 .xcdatamodeld
)文件,并使用自定义 NSEntityMigrationPolicy
适用的对象。
以前,我在对象图上有各种不相关的对象,现在我想要一个主对象Library
,它将使我能够轻松删除所有关联的数据(使用 Cascade删除规则)。
由于我的 NSEntityMigrationPolicy
子类中的自定义方法,我在迁移过程中遇到了问题:
class LegacyToModernPolicy: NSEntityMigrationPolicy {
func libraryForManager(_ manager: NSMigrationManager) -> Library {
let fetchRequest: NSFetchRequest<Library> = NSFetchRequest(entityName: Library.entity().name!)
fetchRequest.predicate = nil
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "filename", ascending: true)]
fetchRequest.fetchLimit = 1
do {
// will fail here if NSFetchRequest<Library>
let results = try manager.destinationContext.fetch(fetchRequest)
log.info("results: \(results)")
if results.count == 1 {
// can fail here if NSFetchRequest<NSManagedObject>
return results.first! as! Library
} else {
let newLib = Library(context: manager.destinationContext)
return newLib
}
} catch {
log.error("Error fetching: \(error.localizedDescription)")
}
let newLib = Library(context: manager.destinationContext)
return newLib
}
}
会抛出异常,错误信息为:
无法将“NSManagedObject_Library_”类型的值 (0x6100000504d0) 转换为“SongbookSimple.Library”(0x101679180)。
问题是,为什么会发生这种情况,这很重要吗?因为正在发生迁移,也许返回具有正确实体描述的 NSManagedObject
就足够了?
最佳答案
原因是在迁移期间,您不应该使用 NSManagedObject 子类的实例。您需要将所有这些都以 NSManagedObject 的形式表达。所以上面的代码必须变成:
class LegacyToModernPolicy: NSEntityMigrationPolicy {
static func find(entityName: String,
in context: NSManagedObjectContext,
sortDescriptors: [NSSortDescriptor],
with predicate: NSPredicate? = nil,
limit: Int? = nil) throws -> [NSManagedObject] {
let fetchRequest: NSFetchRequest<NSManagedObject> = NSFetchRequest(entityName: entityName)
fetchRequest.predicate = predicate
fetchRequest.sortDescriptors = sortDescriptors
if let limit = limit {
fetchRequest.fetchLimit = limit
}
do {
let results = try context.fetch(fetchRequest)
return results
} catch {
log.error("Error fetching: \(error.localizedDescription)")
throw error
}
}
func libraryForManager(_ manager: NSMigrationManager) -> NSManagedObject {
do {
var library: NSManagedObject? = try LegacyToModernPolicy.find(entityName: Library.entity().name!,
in: manager.destinationContext,
sortDescriptors: [NSSortDescriptor(key: "filename", ascending: true)],
with: nil,
limit: 1).first
if library == nil {
let dInstance = NSEntityDescription.insertNewObject(forEntityName: Library.entity().name!, into: manager.destinationContext)
// awakeFromInsert is not called, so I have to do the things I did there, here:
dInstance.setValue(Library.libraryFilename, forKey: #keyPath(Library.filename))
dInstance.setValue(NSDate(timeIntervalSince1970: 0), forKey: #keyPath(Library.updatedAt))
library = dInstance
}
return library!
} catch {
fatalError("Not sure why this is failing!")
}
}}
您可以详细了解我在核心数据迁移方面的不那么有趣的经历 here .
关于ios - 核心数据迁移: Could not cast value of type 'NSManagedObject_MyType' to 'MyModule.MyType' ,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/46174543/