ios之在实体中获取选定的属性_操作系统

我有一个具有多个属性的核心数据实体，我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，或者我应该如何做。我已经阅读了 NSFetchRequest 类的描述，但无法理解其中的意义。

如有任何建议，我们将不胜感激。

编辑:根据 Bluehound 的提示，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，我们将不胜感激。

请您参考如下方法:

有两种可能:你可以发出一个普通的fetch请求并从结果中提取包含所需属性的数组，使用 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)") 
}

swift 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)") 
}

或者您将 resultType 设置为 .DictionaryResultType 和 propertiesToFetch 到想要的属性。在这种情况下，获取请求将返回一个字典数组:

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)") 
}

swift 2:

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只指定属性从数据库中获取，而不是整个托管对象。

缺点是结果不包含pending 托管对象上下文中未保存的更改(includesPendingChanges: 在使用 .DictionaryResultType 时隐式设置为 false。