- (void)setValue:(id)value forUndefinedKey:(NSString *)key
{
   if ([key isEqualToString:@"advert"]) {
      NSArray *bits = [value componentsSeparatedByString:@", "];
      self.rentalPrice = [[bits objectAtIndex:0] floatValue;
      self.address = [bits objectAtIndex:1];
   } else {
      [super setValue:value forUndefinedKey:key];
   }
}

- (id)valueForUndefinedKey:(NSString *)key
{
   if ([key isEqualToString:@"advert"]) {
      return [NSString stringWithFormat:@"%f zł, %@", rentalPrice, address];
   } else {
      [super valueForUndefinedKey:key];
   }
}
