关于ios:Fetching selected attribute in entity

Fetching selected attribute in entities

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：

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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，但无法理解它。

任何建议都将不胜感激。

编辑：收到 Bluehound 的提示后，我将代码更改为：

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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。

有两种可能：可以发出正常的fetch请求
并从结果中提取包含所需属性的数组，
使用 map():

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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:

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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：

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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)")
}

斯威夫特 2:

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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，而不是从整个托管对象中获取。

它的缺点是结果不包含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).

原创文章，作者：ItWorker，如若转载，请注明出处：https://blog.ytso.com/tech/268641.html

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