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，我无法弄清楚为什么，或者我应该如何做到这一点。我已经阅读了NSFetchRequest类的描述，但是没有多大意义。

任何建议都将不胜感激。

编辑：在Bluehound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。

与往常一样，建议将不胜感激。

Answer 1

有两种可能性：您可以发出正常的提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) { $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有指定的属性从数据库中获取，而不是整个托管对象。

它的缺点是结果不包括待处理托管对象上下文中未保存的更改（includesPendingChanges: 使用false时隐式设置为.DictionaryResultType。

获取实体中的选定属性

1 个答案: