核心数据，如何从关系集合中删除一个元素（NSSet）

Question

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：

1. 将关系名称更新为歌曲和播放列表。

2. 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)。

3. 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)
    }
}

Answer 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)
    }
}