Custom NSSortDescriptor on a Complex Object

This is my first post, so I'm sorry if I do not respect all the agreements, although I will try my best. I always found solutions to my problems on SO before, but I was completely stuck on Cocoa's rather complicated problem.

I am trying to do complex sorting in a list of CoreData objects. I have a catalog consisting of book objects that can be part of the Saga (first book and its sequels). Simplified structures look like this:

@interface Book : NSManagedObject
@property (nonatomic, retain) NSString * title;
@property (nonatomic, retain) NSNumber * tomaison; //volume numbering
@property (nonatomic, retain) Saga *fromSaga;

@interface Saga : NSManagedObject
@property (nonatomic, retain) NSString * title;

I am trying to execute a query on my CoreData db, in a book:

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Book" inManagedObjectContext:context];

and I need a view in three stages:

1) Sort by book Genre (not included in the code above, because it is not needed), which is executed using

NSSortDescriptor* mainSort = [[NSSortDescriptor alloc] initWithKey:@"ofGenre.title" ascending:YES selector:@selector(localizedCaseInsensitiveCompare:)];

2) Sorting by the name of the saga, if the book is part of the saga

NSSortDescriptor* secondarySort = [[SagaTitleSortDescriptor alloc] initWithKey:@"fromSaga" ascending:YES];

If a custom sort descriptor is defined as follows:

#define NULL_OBJECT(a) ((a) == nil || [(a) isEqual:[NSNull null]])
@interface SagaTitleSortDescriptor : NSSortDescriptor {}
@end
@implementation SagaTitleSortDescriptor
- (id)copyWithZone:(NSZone*)zone
{
    return [[[self class] alloc] initWithKey:[self key] ascending:[self ascending] selector:[self selector]];
}
- (NSComparisonResult)compareObject:(id)object1 toObject:(id)object2
{
    if (NULL_OBJECT([object1 valueForKeyPath:[self key]])) {
        if (NULL_OBJECT([object2 valueForKeyPath:[self key]])) 
            return NSOrderedSame;
        return NSOrderedDescending;        
    }
    if (NULL_OBJECT([object2 valueForKeyPath:[self key]])) {
        return NSOrderedAscending;
    }
    return [super compareObject:[(Saga*)object1 title] toObject:[(Saga*)object2 title]];
}
@end

3) , , . , , ( , ).

NSSortDescriptor* thirdSort = [[SagaTomaisonOrBookTitleSortDescriptor alloc] initWithKey:@"self" ascending:YES];  

, @ "self" , , , . , :

- (NSComparisonResult)compareObject:(id)object1 toObject:(id)object2
{
    if (NULL_OBJECT([(Book*)object1 fromSaga]) && NULL_OBJECT([(Book*)object2 fromSaga])) {
        return [super compareObject:[(Book*)object1 title] toObject:[(Book*)object2 title]];
    }
    if (NULL_OBJECT([(Book*)object1 fromSaga])) {
        return NSOrderedDescending;
    }
    if (NULL_OBJECT([(Book*)object2 fromSaga])) {
        return NSOrderedAscending;
    }
    return [super compareObject:[(Book*)object1 tomaison] toObject:[(Book*)object1 tomaison]];
}

, ?

!

EDIT: