gistfile1.m

/////// .h ///////

// Core Data MOM file: http://grab.by/jf4c
// NOTE: Some attributes excluded in the code below

@interface CDUserPhotos : NSManagedObject

@property (nonatomic, retain) NSData * data;          // core data attribute
@property (nonatomic, readonly) NSArray* albums;      // not an attribute. Calculate from self.data

@end

/////// .m ///////

@dynamic data;
@synthesize albums;

#pragma mark -
#pragma mark NSManagedObject

- (void)awakeFromFetch
{
    [super awakeFromFetch]; // must be called first
    [self _updateTransient];
}

- (void)didSave
{
    [super didSave];
    [self _updateTransient];
}

- (void)didTurnIntoFault
{
    [super didTurnIntoFault];
    self.albums = nil;
}

- (void)_updateTransient
{
    if ( self.data != nil )
    {
        self.albums = [self.data objectFromJSONData];
    }
    else
    {
        self.albums = nil;
    }
}