Question

Suppose I have a Song entity and Genre entity in Core Data. Genres have many Songs.

I want to perform a fetch request to fetch all songs’ genres. This implies genres would show up multiple times. Or, in SQL terms,

select G.*
  from Songs S
       join Genres G on G.id = S.genre_id

I realise the SQL equivalent is a bit of a stretch since I don’t need foreign/primary keys. The goal is the same. It seems simple, but I couldn’t find an answer in my research that quite fit my problem.

My ultimate goal is to aggregate genres and count them, like in this post. Again, in SQL terms:

select G.name, count(*)
  from Songs S
       join Genres G on G.id = S.genre_id
 group by G.name

I’m hoping that with the understanding from this question, I can add the code from the link to get what I want. Feel free to tell me it won’t work if CD just doesn’t work that way.

Answer 1

Assuming that in your core data model each Genre has a to-many relationship with a Song, the following will give you a count of songs for each genre (which is actually aggregating your genres and counting them), where the genre is attached to at least 1 song:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY songs != NULL"];
NSEntityDescription *genres = [NSEntityDescription entityForName:@"Genre" 
                                          inManagedObjectContext:moc];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:genres];
[request setPredicate:predicate];
NSError *error;
NSArray *genresArray = [moc executeFetchRequest:request error:&error];
NSLog(@"Total number of genres that have songs: %d", genresArray.count);
for (NSManagedObject *genre in genresArray) {
  NSLog(@"Genre: %@ (%@)", [genre valueForKey:@"genreName"], [genre valueForKeyPath:@"songs.@count"]);
}