核心数据，如何从关系集合中删除一个元素（NSSet）
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【中文标题】核心数据，如何从关系集合中删除一个元素（NSSet）【英文标题】：Core Data, how to remove one element from relationship's collection (NSSet) 【发布时间】：2019-09-24 15:51:24 【问题描述】：我有如下多对多核心数据模型，播放列表和歌曲。
我可以成功地将歌曲添加到播放列表的关系（歌曲）中，例如，播放列表1（歌曲1）->添加后，播放列表（歌曲1，歌曲2）。如下代码所示，使用 addToSong(song) 方法，该方法由 CoreData 自动生成，用于将对象添加到关系中。
func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath)
// fetch selected playlist first
let fetchRequest = NSFetchRequest<Playlist>(entityName: "Playlist")
let currentCell = self.tableView.cellForRow(at: indexPath)
let cellText = currentCell?.textLabel?.text
print("cell text", cellText ?? "No Playlist Name")
let predicate = NSPredicate(format: "name = '\(cellText!)' ", "")
fetchRequest.predicate = predicate
let song = NSEntityDescription.insertNewObject(forEntityName: "Song", into: context) as! Song
song.songName = playingSong?.songName
song.artistName = playingSong?.artistName
song.albumName = playingSong?.albumName
song.fileURL = playingSong?.url
print("song name", playingSong?.songName ?? "no songName")
do
let selectedPlaylists = try self.context.fetch(fetchRequest)
for item in selectedPlaylists
item.addToSong(song)
catch let error as NSError
print("Could not delete. \(error), \(error.userInfo)")
navigationController?.popViewController(animated: true)
// show short alert message to user
showAlert(userMessage: "Song added")
但是当我尝试从播放列表的关系中删除一些歌曲时，使用 removeFromSong(song)，它不起作用。就我而言，我想做的是，例如之前，playlist1（song1，song2，song3）和 removeFromSong（xx）之后，playlist1（song2，song3）。我确实搜索了网络，但没有找到如何定位要从关系中删除的特定对象，任何帮助都将不胜感激！
///在下面的代码中，我创建了一个新的Song对象，并为其分配了两个属性songName和artistName，然后使用removeFromSong()将这首新创建的歌曲从播放列表的关系中删除，但我不知道是不是这样可以找到保存在关系中的正确歌曲。
func deleteSong(indexPath: IndexPath)
// remove record from Playlist entity of DB
let fetchRequest = NSFetchRequest<Playlist>(entityName: "Playlist")
let currentCell = self.tableView.cellForRow(at: indexPath)
// let cellText = currentCell?.textLabel?.text
let cellText = navigationItem.title
let predicate = NSPredicate(format: "name = '\(cellText!)' ", "")
fetchRequest.predicate = predicate
let song = NSEntityDescription.insertNewObject(forEntityName: "Song", into: context) as! Song
song.songName = currentCell?.textLabel?.text
song.artistName = currentCell?.detailTextLabel?.text
do
let selectedPlaylists = try self.context.fetch(fetchRequest)
for item in selectedPlaylists
// delete selected song in current playlist
item.removeFromSong(song)
// item.objectIDs(forRelationshipNamed: <#T##String#>)
// save the changes after deleting
try context.save()
catch let error as NSError
print("Could not delete. \(error), \(error.userInfo)")
// remove data from tableView
tableView.deleteRows(at: [indexPath], with: UITableView.RowAnimation.automatic)
//refresh tableView
tableView.reloadData()
///作为vadian的评论，将我的代码更改如下。
变化：
将关系名称更新为歌曲和播放列表。
删除insertNewObject的代码模式，现在首先从索尼实体获取/查找所选歌曲。然后从播放列表中删除这首歌removeFromSong(song)。
去掉reloadData，deleteRow后面不需要出现。
问题： 现在我可以通过滑动删除从 tableView 中删除所选内容，但如果我强制关闭应用程序或导航到其他视图并返回，删除项将被支持。所以删除在核心数据模型中不受影响。当我使用下面的代码（例如添加歌曲）时，效果很好，所以删除的问题在哪里，任何提示都是合适的。
do
let selectedPlaylists = try self.context.fetch(fetchRequest)
for item in selectedPlaylists
// delete selected song in current playlist
print("Ready to remove!!!!!")
item.removeFromSongs(selectedSong[0])
// save the changes after deleting
try self.context.save()
catch let error as NSError
print("Could not delete. \(error), \(error.userInfo)")
///完整代码：
import UIKit
import CoreData
class ShowPlaylistDetailsViewController: UITableViewController
var song: SongData?
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
var playlistObjects = [Playlist]()
var songObjects = [Song]()
var playlistName = ""
var selectedSong = [Song]()
// var rowCount: Int?
override func viewDidLoad()
super.viewDidLoad()
override func viewWillAppear(_ animated: Bool)
super.viewWillAppear(animated)
// set navigation controller's title
navigationItem.title = playlistName
// print("playlist name", playlistName)
let fetchRequest = NSFetchRequest<Playlist>(entityName: "Playlist")
let predicate = NSPredicate(format: "name = '\(playlistName)' ", "")
fetchRequest.predicate = predicate
do
playlistObjects = try context.fetch(fetchRequest)
catch
fatalError("Can not query: \(error)")
songObjects = playlistObjects[0].songs?.allObjects as! [Song]
// refresh table data.
tableView.reloadData()
// MARK: - Table view data source
override func numberOfSections(in tableView: UITableView) -> Int
// #warning Incomplete implementation, return the number of sections
return 1
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int
// print("songs count", songsCount!)
return songObjects.count
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell
let cell = tableView.dequeueReusableCell(withIdentifier: "showPlaylistDetails", for: indexPath)
cell.textLabel?.text = songObjects[indexPath.row].songName
cell.detailTextLabel?.text = songObjects[indexPath.row].artistName
cell.imageView?.image = UIImage(named: "icons8-music-50")
cell.imageView?.layer.cornerRadius = 0.8
return cell
override func tableView(_ tableView: UITableView, trailingSwipeActionsConfigurationForRowAt indexPath: IndexPath) -> UISwipeActionsConfiguration?
let deleteAction = UIContextualAction(style: .normal, title: "Delete", handler: (action, view, completion) in
self.deleteSong(indexPath: indexPath)
)
// action.image = UIImage(named: "My Image")
deleteAction.backgroundColor = .red
let swipeActions = UISwipeActionsConfiguration(actions: [deleteAction])
swipeActions.performsFirstActionWithFullSwipe = false
return swipeActions
func deleteSong(indexPath: IndexPath)
// find current selected cell
let currentCell = self.tableView.cellForRow(at: indexPath)
// find the deleted song
let fetchRequestForSong = NSFetchRequest<Song>(entityName: "Song")
let ssName = currentCell?.textLabel?.text
print("ssName", ssName ?? "Song name retrieve failed")
let predicateForSong = NSPredicate(format: "songName = '\(ssName ?? "Song name retrieve failed")' ", "")
fetchRequestForSong.predicate = predicateForSong
do
selectedSong = try self.context.fetch(fetchRequestForSong)
// print("songName", selectedSong.first?.songName)
catch let error as NSError
print("Could not delete. \(error), \(error.userInfo)")
// find current selected playlist
let fetchRequest = NSFetchRequest<Playlist>(entityName: "Playlist")
let cellText = navigationItem.title
let predicate = NSPredicate(format: "name = '\(cellText!)' ", "")
fetchRequest.predicate = predicate
do
let selectedPlaylists = try self.context.fetch(fetchRequest)
for item in selectedPlaylists
// delete selected song in current playlist
print("Ready to remove!!!!!")
item.removeFromSongs(selectedSong[0])
// save the changes after deleting
try self.context.save()
catch let error as NSError
print("Could not delete. \(error), \(error.userInfo)")
// remove song from dataSource array
songObjects.remove(at: indexPath.row)
// remove data from tableView
tableView.deleteRows(at: [indexPath], with: UITableView.RowAnimation.automatic)
@IBAction func naviBack(_ sender: UIBarButtonItem)
navigationController?.popViewController(animated:true)
【问题讨论】：
您的代码非常混乱。首先，您应该以复数形式命名对多关系：songs 和 playlists。您正在deleteSong 中插入一首新歌曲。这行不通。您必须获取给定名称和艺术家的歌曲并将其从您想要的播放列表中删除。而且永远不会从单元格中获取静态数据，而是从数据模型中获取。并且永远不要在deleteRows 之后立即调用reloadData。
@vadian，我根据您的建议更新了我的代码，插入部分确实是错误的......当时我的大脑很乱。好吧，还有一些问题，比如 removeFromSong() 并没有真正从播放列表的关系中删除那首歌，你能帮忙吗？
我还是一头雾水。尽管可以从具有给定索引（路径）的 songObjects 数组中获取歌曲，但为什么要获取歌曲？谓词格式字符串看起来很奇怪。为什么不是标准的NSPredicate(format: "name == %@", cellText) 语法？在 do catch 块之后执行代码是非常糟糕的做法，就好像永远不会发生错误一样。我不知道removeFromSongs 做了什么，但似乎它确实从关系中删除了这首歌。再说一次，不要从单元格标签中获取任何数据，而是从songObjects 中的项目中获取数据。
【参考方案1】：
根据我的 cmets，这是删除方法的清理版本，如果 removeFromSongs 确实从关系中删除了歌曲
func deleteSong(at indexPath: IndexPath)
// get current selected song
let currentSong = songObjects[indexPath.row]
// find current selected playlist
let fetchRequest = NSFetchRequest<Playlist>(entityName: "Playlist")
let cellText = navigationItem.title
let predicate = NSPredicate(format: "name == %@", cellText!)
fetchRequest.predicate = predicate
fetchRequest.fetchLimit = 1
do
if let selectedPlaylist = try self.context.fetch(fetchRequest).first
print("Ready to remove!!!!!")
selectedPlaylist.removeFromSongs(currentSong)
// save the changes after deleting
try self.context.save()
// remove song from dataSource array
songObjects.remove(at: indexPath.row)
// remove data from tableView
tableView.deleteRows(at: [indexPath], with: .automatic)
catch
print("Could not delete.", error)
【讨论】：
谢谢，瓦迪安。我像你一样更改我的代码，现在删除对核心数据模型产生影响。是的，当使用do catch 时，我忘记在let, try 之前添加if，并且我更改了其他格式问题，例如NSPredicate，您的回答确实对我有帮助，我认为可能有很多这样的格式问题我的项目:)以上是关于核心数据，如何从关系集合中删除一个元素（NSSet）的主要内容，如果未能解决你的问题，请参考以下文章
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