ios: coreData的NSManagedObject setvalue为null

当用到coredata的时候，当你存数据：

    NSManagedObject *oneObject = [NSEntityDescription
                 insertNewObjectForEntityForName:@"DrawData"
                 inManagedObjectContext:managedObjectContext];
    
    [oneObject setValue:data.backGroundImageURL forKey:@"backGroundImageURL"];
    [oneObject setValue:data.drawImagePath forKey:@"drawImagePath"];
    [oneObject setValue:data.finalImagePath forKey:@"finalImagePath"];

若此时有 backGroundImageURL的值为null，而 drawImagePath 及  finalImagePath 的值不为空。则在取出coredata的数据时，所有数据（backGroundImageURL，drawImagePath，finalImagePath）都会是null的。