Cocoa Touch,核心数据:链接器错误,未找到NSManagedObject符号

我有一个cocoa touch应用程序,使用核心数据框架。

我创建了一个包含两个实体的xcdatamodel: Program 和 ProgramReplay .
节目回放 与…有关系 程序 ,后者中存在反向关系。

我保存了这个,并使用XCode的特性为这两个实体生成了类。生成的标题如下所示:

#import <CoreData/CoreData.h>
@class ProgramReplay;
@interface Program :  NSManagedObject  
{
}

// other properties here, removed them to keep it short
@property (nonatomic, retain) NSSet* replays;
@end

@interface Program (CoreDataGeneratedAccessors)
- (void)addReplaysObject:(ProgramReplay *)value;
- (void)removeReplaysObject:(ProgramReplay *)value;
- (void)addReplays:(NSSet *)value;
- (void)removeReplays:(NSSet *)value;
@end

#import <CoreData/CoreData.h>
@class Program;
@interface ProgramReplay :  NSManagedObject  
{
}

@property (nonatomic, retain) NSDate * date;
@property (nonatomic, retain) Program * program;
@end

".objc_class_name_NSManagedObject", referenced from:
.objc_class_name_Program in Program.o
.objc_class_name_ProgramReplay in ProgramReplay.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

我做错了什么?