Question

I’m working on a Core Data based application (I’m used to work with sqlite, but this time I wanted to learn something new) and I’m stuck on this relation problem:

I have Songs and Playlists, and I want to relate them with an order number, so playlists can be like this:

 - "Playlist A"   
       1. Song A  
       2. Song C  
       3. Song B
 - "Playlist B" 
       1. Song C  
       2. Song A  
       3. Song B
       4. Song F
       5. Song E

This is an obvious many-2-many case, with (i know it’s not standard) the order number stored in related table; in sql I’d do

  Table  "Songs"  (id, name, ecc..)
  Table  "Playlists" (id,name, ecc..)
  Table  "PlaylistSongs" (playlist_id,song_id,position)

In Core Data i did:
      Entity "Song"
      Entity "Playlist"
      Entity "SongInPlaylist"

with this relation:
  Song <---->> SongInPlaylist
  Playlist <----->> SongInPlaylist

But now i don’t know how to:

a) put a song in playlist at certain index
b) get songs in a playlist in correct order
c) modify playlist order.

Does anybody can help me?

Answer 1

It seems that a SongInPlaylist needs a position attribute. You can then give the PlayList some methods to insert at an index, move from one index to another, or remove at an index which would fix the position values of other SongInPlaylist entities.