Sort NSArray with custom objects

In my Xcode project I have the following classes: Address ``` @interface LDAddress : NSObject{ NSString *street; NSString *zip; NSString *city; float latitude; float longitude; } @property (nonatomic, retain) NSString *street; @property (nonatomic, retain) NSString *zip; @property (nonatomic, retain) NSString *city; @property (readwrite, assign, nonatomic) float latitude; @property (readwrite, assign, nonatomic) float longitude; @end ``` Location ``` @interface LDLocation : NSObject{ int locationId; NSString *name; LDAddress *address; } @property (readwrite, assign, nonatomic) int locationId; @property (nonatomic, retain) LDAddress *address; @property (nonatomic, retain) NSString *name; @end ``` In an subclass of UITableViewController, there is a NSArray containing a lot of unsorted objects of LDLocations. Now, I want to sort the NSArray's objects ascending based on the property city of a LDAddress. How can I sort the array using NSSortDescriptor? I tried the following, but the app dumps when sorting the array. ``` NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"Address.city" ascending:YES]; [_locations sortedArrayUsingDescriptors:@[sortDescriptor]]; ```