Question

I’m new to Core Data.

In the Data Model I have the entities:

• Song
• Playlist

This is the relationship:

• Every Song belongs to no, one or multiple Playlist‘s

The I’ve added the relationship to the model like this.
I also made the inverse To-Many Relationship.

Now my question.

The Playlist‘s now have a NSSet with all the Songs.
So I can access them via playlist.songs.

I can fetch any specific Song using a NSPredicate:

- (NSArray *)fetchWithEntity:(NSString *)entity
                   predicate:(NSPredicate *)predicate
             sortDescriptors:(NSArray *)sortDescriptors
                       error:(NSError **)error {

    NSManagedObjectContext *moc = [self managedObjectContext];
    NSEntityDescription *entityDescription = [NSEntityDescription
                                              entityForName:entity
                                              inManagedObjectContext:moc];

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setEntity:entityDescription];
    [request setPredicate:predicate];
    [request setSortDescriptors:sortDescriptors];

    NSArray *fetchResult = [moc executeFetchRequest:request error:error];
    return fetchResult;
}

However, I’d like to be possible to filter the Songs of a specific playlist too, rather than having to filter the NSSet with an NSPredicate, because it’s simply faster.

How can I do this?

Answer 1

The get songs of a specific playlist, just add "playlist = %@" to the predicate used for fetching Song objects, e.g.

[NSPredicate predicateWithFormat:@"playlist = %@ AND title CONTAINS[cd] %@",
             thePlaylist, songTitle]

Or, if the playlist is given by a title:

[NSPredicate predicateWithFormat:@"playlist.title = %@ AND title CONTAINS[cd] %@",
             playlistTitle, songTitle]

EDIT If there is a many-to-many relationship between Songs and Playlist, you can define playlists as to-many relationship from Song to Playlist, as inverse relationship to songs. Then the following predicate should work:

[NSPredicate predicateWithFormat:@"ANY playlist = %@ AND title CONTAINS[cd] %@",
             thePlaylist, songTitle]