Question

How to use fetched property to create sorted array? I am new to predicate programming.

Say there are two entities: ‘MyEntity’ and ‘Image’. ‘MyEntity’ have a to-many relationship to ‘Image’ called ‘images’.
‘Image’ have two attributes: ‘image’ and ‘index’ where I want to use ‘index’ as sort key to sort images.

I want to have a fetched property ‘sortedImages’ on ‘MyEntity’ which will return a sorted version of ‘images’.

What I used to do is load all of the images and sort them in memory and store the result but I found this may consume too much memory.

- (NSArray *)sortedImages {
    if (!sortedImages) {
        NSSortDescriptor *descriptor = [NSSortDescriptor sortDescriptorWithKey:@"index" ascending:YES];
        sortedImages = [self.images sortedArrayUsingDescriptors:[NSArray arrayWithObject:descriptor]];
        [sortedImages retain];
    }
    return sortedImages;
}

Answer 1

Not sure if a fetchedProperty is the right solution here, but you can create an NSFetchRequest on the managedObjectContext of the parent entity and use an NSSortDescriptor on the key, “index”. That should return a “fault-capable” sorted array of image objects.