要使用这个属性一定要注意先把数据保存下，不然会变化的！就无法通过
- (NSManagedObject*)existingObjectWithID:(NSManagedObjectID*)objectID error:(NSError**)error NS_AVAILABLE(10_6, 3_0);
方法取得对象了，下面是这个属性的英文介绍：
IMPORTANT
If the receiver has not yet been saved, the object ID is a temporary value that will change when the object is saved.