我有一个具有多个属性的核心数据实体,并且想要一个属性中所有对象的列表。我的代码如下所示:
let appDel:AppDelegate = UIApplication.sharedApplication().delegate as AppDelegate let context:NSManagedObjectContext = appDel.managedObjectContext! let sortDesc = NSSortDescriptor(key: "username", ascending: true) let fetchReq = NSFetchRequest(entityName: "Identities") fetchReq.sortDescriptors = [sortDesc] fetchReq.valueForKey("username") let en = NSEntityDescription.entityForName("Identities", inManagedObjectContext: context) userList = context.executeFetchRequest(fetchReq, error: nil) as [Usernames]
但这给了我一个NSException错误,我不知道为什么,或者我应该怎么做。我已经阅读了NSFetchRequest类的描述,但从中讲不出来。
任何建议,将不胜感激。
编辑:从Bluehound获得提示后,我将代码更改为:
var userList = [Model]() @IBAction func printUsers(sender: AnyObject) { let appDel:AppDelegate = UIApplication.sharedApplication().delegate as AppDelegate let context:NSManagedObjectContext = appDel.managedObjectContext! let sortDesc = NSSortDescriptor(key: "friendID", ascending: true) let fetchReq = NSFetchRequest(entityName: "Identities") fetchReq.sortDescriptors = [sortDesc] fetchReq.propertiesToFetch = ["friendID"] let en = NSEntityDescription.entityForName("Identities", inManagedObjectContext: context) userList = context.executeFetchRequest(fetchReq, error: nil) as [Model] println(userList) }
运行时错误消失了,但我仍然不知道它是否有效,因为我不确定如何将列表转换为字符串列表。
一如既往,建议将不胜感激。
有两种可能:您可以发出普通的提取请求,并使用map()以下命令从结果中提取包含所需属性的数组:
map()
let fetchReq = NSFetchRequest(entityName: "Identities") fetchReq.sortDescriptors = [sortDesc] var error : NSError? if let result = context.executeFetchRequest(fetchReq, error: &error) as? [Model] { let friendIDs = map(result) { $0.friendID } println(friendIDs) } else { println("fetch failed: \(error!.localizedDescription)") }
斯威夫特2:
let fetchReq = NSFetchRequest(entityName: "Identities") fetchReq.sortDescriptors = [sortDesc] do { let result = try context.executeFetchRequest(fetchReq) as! [Model] let friendIDs = result.map { $0.friendID } print(friendIDs) } catch let error as NSError { print("fetch failed: \(error.localizedDescription)") }
或者你设置resultType到 .DictionaryResultType 和propertiesToFetch到想要的属性。在这种情况下,提取请求将返回一个字典数组:
resultType
.DictionaryResultType
propertiesToFetch
let fetchReq = NSFetchRequest(entityName: "Identities") fetchReq.sortDescriptors = [sortDesc] fetchReq.propertiesToFetch = ["friendID"] fetchReq.resultType = .DictionaryResultType var error : NSError? if let result = context.executeFetchRequest(fetchReq, error: &error) as? [NSDictionary] { let friendIDs = map(result) { $0["friendID"] as String } println(friendIDs) } else { println("fetch failed: \(error!.localizedDescription)") }
let fetchReq = NSFetchRequest(entityName: "Identities") fetchReq.sortDescriptors = [sortDesc] fetchReq.propertiesToFetch = ["friendID"] fetchReq.resultType = .DictionaryResultType do { let result = try context.executeFetchRequest(fetchReq) as! [NSDictionary] let friendIDs = result.map { $0["friendID"] as! String } print(friendIDs) } catch let error as NSError { print("fetch failed: \(error.localizedDescription)") }
第二种方法的优点是,仅从数据库中获取指定的属性,而不是整个托管对象。
这样做的缺点是,结果不包括托管对象上下文中未决的未保存更改(使用时includesPendingChanges: 隐式设置为)。false``.DictionaryResultType
includesPendingChanges:
false``.DictionaryResultType