Hi there 👋

Problem Overview While developing an iOS music player app, I encountered an issue where adding a song to a new playlist inadvertently removed it from its original playlist. This occurred due to incorrect management of bi-directional relationships between songs and playlists. The relationship setup: A MusicModel represents a song and has a playlists property linking to the playlists it belongs to. A PlaylistModel represents a playlist and has a songs property linking to its songs. Original Code Adding Songs to a Playlist private func addSelectedSongs() { let songsToAdd = allSongs.filter { selectedSongs.contains($0.id) } for song in songsToAdd { playlist.songs.append(song) song.playlists.append(playlist) } playlist.updatedAt = Date() try? modelContext.save() } Removing Songs from a Playlist private func removeSong(_ song: MusicModel) { playlist.songs.removeAll { $0.id == song.id } song.playlists.removeAll { $0.id == playlist.id } playlist.updatedAt = Date() try? modelContext.save() } Issue with the Original Code Direct Modification of Relationships: Adding a playlist to a song (song.playlists.append(playlist)) unintentionally overwrote existing playlist associations. Updated Code Updated addSelectedSongs private func addSelectedSongs() { let songsToAdd = allSongs.filter { selectedSongs.contains($0.id) } for song in songsToAdd { // Ensure the song is not already in the playlist if !playlist.songs.contains(where: { $0.id == song.id }) { playlist.songs.append(song) } // Ensure the playlist is not already in the song if !song.playlists.contains(where: { $0.id == playlist.id }) { song.playlists.append(playlist) } } playlist.updatedAt = Date() try? modelContext.save() } Updated removeSong private func removeSong(_ song: MusicModel) { // Remove the song from the current playlist playlist.songs.removeAll { $0.id == song.id } // Remove the current playlist from the song song.playlists.removeAll { $0.id == playlist.id } playlist.updatedAt = Date() try? modelContext.save() } Idea to fix the issue Preserves Existing Associations: The updated addSelectedSongs method checks if the song or playlist already exists in the relationship before appending. This prevents overwriting existing associations. Scoped Removal: The updated removeSong method only removes the current playlist from the song’s playlists, ensuring other playlist associations remain intact. Ensures Data Integrity: The changes ensure that a song can belong to multiple playlists simultaneously without any unintended side effects.