JSONModel로 직렬화하려고 시도한 후에 오류가 발생했습니다. JSON 쓰기 (...)가 잘못된 유형입니다.

프레임 워크 JSOSNModel로 객체를 직렬화하려고합니다.JSONModel로 직렬화하려고 시도한 후에 오류가 발생했습니다. JSON 쓰기 (...)가 잘못된 유형입니다.

buchung.m

-(NSMutableArray *) FetchDienstleistungenImWarenkorb 
{ 
    NSFetchRequest *request = [[NSFetchRequest alloc] init]; 
    NSManagedObjectContext *context = [app managedObjectContext]; 
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Warenkorb" inManagedObjectContext:context]; 

    [request setEntity:entity]; 

    NSSortDescriptor *sort1 = [[NSSortDescriptor alloc] initWithKey:@"vondatum" ascending:YES]; 
    NSArray *sortArray = [NSArray arrayWithObjects:sort1, nil]; 

    [request setSortDescriptors:sortArray]; 

    NSError *error; 
    NSArray *fetchedObjects = [[app managedObjectContext]executeFetchRequest:request error:&error]; 

    if (fetchedObjects == nil) { 
     NSLog(@"Houston, we have a problem: %@", error); 
    } 

    DienstleistungModel *dm = [[DienstleistungModel alloc]init]; 

    NSMutableArray *produkt = [[NSMutableArray alloc]init]; 

    for (NSArray *event in fetchedObjects) { 
     dm.dienstleistungid = [event valueForKey:@"produktid"]; 
     dm.vondatum = [event valueForKey:@"vondatum"]; 
     dm.bisdatum = [event valueForKey:@"bisdatum"]; 
     dm.menge = [event valueForKey:@"menge"]; 
     [produkt addObject:dm]; 
     } 
      return product;   // Here iam getting a list of products, saved in a mutable array 

} 


(IBAction)nextPressed:(id)sender { 

    Booking *aktuellebuchung = [[Booking alloc]init]; 
    aktuellebuchung.bestuhlungsid = @"3"; 
    aktuellebuchung.vondatum = self.vonDatumLabel.text; 
    aktuellebuchung.bisdatum = self.bisDatumLabel.text; 
    aktuellebuchung.thema = self.themaTextView.text; 
    aktuellebuchung.personenanzahl = self.anzahlPersonenLabel.text; 
    aktuellebuchung.veranstalter = self.veranstalterLabel.text; 
    aktuellebuchung.dienstetest = [self FetchDienstleistungenImWarenkorb]; 

    NSString *test = [aktuellebuchung toJSONString]; // Here is the error

booking.h

@interface Booking : JSONModel 
@property (nonatomic, retain) NSString * bestuhlungsid; 
@property (nonatomic, retain) NSString * bisdatum; 
@property (nonatomic, retain) NSString * vondatum; 
@property (nonatomic, retain) NSString * personenanzahl; 
@property (nonatomic, retain) NSString * thema; 
@property (nonatomic, retain) NSString * veranstalter; 
@property (nonatomic, retain) NSMutableArray * dienstetest; 

@end

DienstleistungModul.h

여기

[JSONModel.m:915] EXCEPTION: Invalid type in JSON write (DienstleistungModel)

내 소스 코드입니다 :하지만 다음과 같은 오류를 얻고있다

@protocol DienstleistungModel @end 
@interface DienstleistungModel : JSONModel 
@property (nonatomic, retain) NSNumber * dienstleistungid; 
@property (nonatomic, retain) NSString * bisdatum; 
@property (nonatomic, retain) NSString * vondatum; 
@property (nonatomic, retain) NSNumber * menge; 
@end

screenshot