在实体中获取选定的属性

Question

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 class 的描述，但无法理解它。

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。

Answer 1

有两种可能：您可以发出正常的获取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) { [=10=].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 { [=11=].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 return 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) { [=12=]["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 { [=13=]["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: 使用 .DictionaryResultType).

时隐式设置为 false

在实体中获取选定的属性

Fetching selected attribute in entities

core-data

nsfetchrequest

nsmanagedobjectcontext

ios

swift