Using MPMediaQuery to query ios library by rating - ios

Question

I'm developing an iphone 4 app that filters the local media library by different criteria. I have been able to filter by song name, artist, and genre. Now I need to add a rating filter mechanism. I was thinking in something like "rating >= N stars" (N:0..5)
The code I use to filter is:
allMedia = [MPMediaQuery songsQuery];
MPMediaPropertyPredicate *mpp1 = [MPMediaPropertyPredicate predicateWithValue:#"2" forProperty:MPMediaItemPropertyRating comparisonType:MPMediaPredicateComparisonEqualTo];
[allMedia addFilterPredicate:mpp1];
But MPMediaPropertyPredicate does not allow to filter by MPMediaItemPropertyRating (and actually it works ok with artist and song title).
2011-05-12 11:37:39.060 Radio3[1525:707] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'MPMediaPropertyPredicate cannot filter using the rating property.'
I google by MPMediaItemPropertyRating but it seems that I should find an alternative approach to filter by track rating.
Can somebody give me a tip?
thanks
UPDATE: This is my code to solve this:
allMedia = [MPMediaQuery songsQuery];
//MPMediaPropertyPredicate *mpp1 = [MPMediaPropertyPredicate predicateWithValue:#"2" forProperty:MPMediaItemPropertyRating comparisonType:MPMediaPredicateComparisonEqualTo];
//MPMediaPropertyPredicate *mpp2 = [MPMediaPropertyPredicate predicateWithValue:#"Pop" forProperty:MPMediaItemPropertyGenre comparisonType:MPMediaPredicateComparisonContains];
//[allMedia addFilterPredicate:mpp1];
//[allMedia addFilterPredicate:mpp2];
//[myPlayer setQueueWithQuery:allMedia];
NSArray *itemsFromGenericQuery = [allMedia items];
NSMutableArray *mArray = [[NSMutableArray alloc] init];
int i = 0;
int j=0;
NSLog(#"itemCount: %d",[itemsFromGenericQuery count]);
float playsQuery = sliderPlays.value;
if(playsQuery == 20){playsQuery = 10000;}
NSLog(#"sliderRating.value %f sliderPlays.value %.1f", [sliderRating value], playsQuery);
while(i++ < 1000){
int trackNumber = arc4random() % [itemsFromGenericQuery count];
MPMediaItem *song = [itemsFromGenericQuery objectAtIndex:trackNumber];
NSString *artistName = [song valueForProperty: MPMediaItemPropertyArtist];
NSString *title = [song valueForProperty: MPMediaItemPropertyTitle];
NSString *rating = [song valueForKey:MPMediaItemPropertyRating];
double lengh = [[song valueForProperty:MPMediaItemPropertyPlaybackDuration] doubleValue];
NSNumber *playCount = [song valueForKey:MPMediaItemPropertyPlayCount];
if ([rating intValue] >= sliderRating.value && [playCount intValue] <= playsQuery) {
if(j++ > 50){break;}
NSLog (#"tracknumber: %d j: %d artistName: %# title: %# lengh: %.1f rating: %# playcount: %d",trackNumber, j, artistName, title, lengh, rating, [playCount intValue]);
[mArray addObject:song];
}
if(i++ > 1000)break;
}
MPMediaItemCollection *itemCol = [[MPMediaItemCollection alloc] initWithItems:mArray];
[myPlayer setQueueWithItemCollection:itemCol];
[myPlayer setShuffleMode: MPMusicShuffleModeSongs];
[myPlayer setRepeatMode: MPMusicRepeatModeNone];

Answer 1

MPMediaItemPropertyRating is a user-defined property, and according to the Apple docs:
User-defined properties cannot be used to build media property predicates.
One way to get around this would be to initially iterate over all the songs, store the ratings in a database (or something else) and sort the data from there.