iOS 沙盒中copy的问题

我有一个.db的数据库 我copy到沙盒中之后用BOOL 判断copy成功 但是内容全不见了 ？
0KB???
NSFileManager *fieldManager=[NSFileManager defaultManager];

    NSString *nfieldPath=[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask,YES)[0]stringByAppendingPathComponent:@"number_location.db"];

    NSString *yfieldPath=[[NSBundle mainBundle] pathForResource:@"number_location" ofType:@"db"];

    if(![fieldManager fileExistsAtPath:nfieldPath])
    {
        BOOL isCopy=[fieldManager copyItemAtPath:yfieldPath toPath:nfieldPath error:nil];
        if(isCopy)
        {
            NSLog(@"复制成功");

        }
    }