获取实体中的选定属性 [英] Fetching selected attribute in entities

问题描述

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:

I have a core-data entity with several attributes, and I want a list of all the objects in one attribute. My code looks like this:

        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]

但是这给了我一个 NSException 错误，我不知道为什么，或者我应该如何做到这一点.我已经阅读了 NSFetchRequest 类的描述，但对它没有多大意义.

But this gives me an NSException-error, and I can't figure out why, or how I'm supposed to do this. I've read the NSFetchRequest class description but couldn't make much sense out of it.

如有任何建议，我们将不胜感激.

Any suggestions would be appreciated.

在 Bluehound 的提示后，我将代码更改为:

After a tip from Bluehound I changed my code to this:

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.

The runtime error is gone but I still don't know if it works because I'm not sure how to convert the list to a list of strings.

一如既往，我们将不胜感激.

As always, suggestions would be appreciated.

推荐答案

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():

There are two possibilities: You can issue a normal fetch request and extract an array containing the wanted attribute from the result, using 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)")
}

或者你将 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:

Or you set the resultType to .DictionaryResultType and propertiesToFetch to the wanted attribute. In this case the fetch request will return an array of dictionaries:

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是从数据库中获取的，而不是整个托管对象.

The second method has the advantage that only the specified properties are fetched from the database, not the entire managed objects.

它的缺点是结果不包括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).

It has the disadvantage that the result does not include pending unsaved changes in the managed object context (includesPendingChanges: is implicitly set to false when using .DictionaryResultType).

这篇关于获取实体中的选定属性的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！