problem with accessing attributes of relationship after fetching with setPropertiesToFetch

I'm only fetching some attributes and a 1-to-1 relationship of an entity using -setPropertiesToFetch and result type set to NSDictionaryResultType. Now I have a problem accessing the attributes of the returned relationship. As soon as I want to access a property I get an NSInvalidArgumentException', reason: unrecognized selector sent to instance

Here is the full Exception:

2011-03-23 11:02:10.435 ThurboApp[32996:207] -[_NSObjectID_48_0 lon]: unrecognized selector sent to instance 0x5a3f490
2011-03-23 11:02:10.441 ThurboApp[32996:207] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[_NSObjectID_48_0 lon]: unrecognized selector sent to instance 0x5a3f490'

Here is the corresponding source code:

    - (void)fetchAllPois
{
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"POI" inManagedObjectContext:self.managedObjectContext];
    request.entity = entity;
    [request setResultType:NSDictionaryResultType];
    [request setPropertiesToFetch:[NSArray arrayWithObjects:@"poiId",@"categoryId",@"poiTitle",@"coordinates", nil]];
    request.sortDescriptors = nil;
    request.predicate = nil;

    NSError *error = nil;
    self.fetchResult = [self.managedObjectContext executeFetchRequest:request error:&error];

}


- (MapPoint *)createMapPointFromDictionary:(NSDictionary *)dict
{
    NSString *poiId = [dict objectForKey:@"poiId"];
    NSString *title = [dict objectForKey:@"poiTitle"];
    Coordinates *coords = (Coordinates *)[dict objectForKey:@"coordinates"];
    NSNumber *category开发者_Python百科 = [dict objectForKey:@"categoryId"];
    CLLocationCoordinate2D coordinates = CLLocationCoordinate2DMake([coords.lat doubleValue], [coords.lon doubleValue]); //here raises the exception
    MapPoint *p = [[[MapPoint alloc] initWithPoiId:poiId title:title category:[category intValue] coordinates:coordinates] autorelease];
    return p;

}

interface for Coordinates:

@interface Coordinates :  NSManagedObject  
{
}

@property (nonatomic, retain) NSNumber * lat;
@property (nonatomic, retain) NSNumber * alt;
@property (nonatomic, retain) NSNumber * lon;
@property (nonatomic, retain) NSManagedObject * poi;

@end

I've already checked, that the values returned in the dictionaries are correct. Even coordinates points to a Coordinate object.

Help is much appreciated.

Ok I managed to solve the problem. What you get for the relationship is a NSManagedObjectID which you can use with objectWithID to get the Entity itself. Here is the updated code:

- (MapPoint *)createMapPointFromDictionary:(NSDictionary *)dict
{
    NSString *poiId = [dict objectForKey:@"poiId"];
    NSString *title = [dict objectForKey:@"poiTitle"];
    NSManagedObjectID *coordsID = (NSManagedObjectID *)[dict objectForKey:@"coordinates"]; //get the objectid
    NSNumber *category = [dict objectForKey:@"categoryId"];
    Coordinates *coords = (Coordinates *)[self.managedObjectContext objectWithID:coordsID]; //get the actual managedobject
    CLLocationCoordinate2D coordinates = CLLocationCoordinate2DMake([coords.lat doubleValue], [coords.lon doubleValue]);
    MapPoint *p = [[[MapPoint alloc] initWithPoiId:poiId title:title category:category coordinates:coordinates] autorelease];
    return p;

}