为什么用户的脸书ID在ios中发生了变化？

Question

我是整合Facebook的新手。到目前为止，我已经实现了登录。我一直在我们的数据库中保存用户ID，作为一种在登录时识别它们的方法。但是我今天注意到id改变了。如果id发生变化，我可以用什么来识别facebook用户？

以下是我正在使用的代码，g_fb_id是传递给我们服务器的ID。

// This method will handle ALL the session state changes in the app
- (void)sessionStateChanged:(FBSession *)session state:(FBSessionState) state error:(NSError *)error
{
    // If the session was opened successfully
    if (!error && state == FBSessionStateOpen){
        NSLog(@"Session opened");

        // get facebook ID
        [[FBRequest requestForMe] startWithCompletionHandler:^(FBRequestConnection *connection, NSDictionary<FBGraphUser> *user, NSError *error) {
            if (! error) {
                g_fb_id = [user objectForKey:@"id"];
                g_user.birthday=user.birthday;
                g_user.fb_page=[user objectForKey:@"link"];
                g_user.first_name=user.first_name;
                g_user.last_name=user.last_name;
                g_user.email=[user objectForKey:@"email"];
                g_user.gender=[user objectForKey:@"gender"];



                [self fbLoggedIn];
            } else {
                [self fbLoginFailed:@"Login failed !!!"];
            }
        }];
    } else if (state == FBSessionStateClosed || state == FBSessionStateClosedLoginFailed){
        // If the session is closed
    } else {
        // handle errors
        // Clear this token
        [FBSession.activeSession closeAndClearTokenInformation];
        [self fbLoginFailed:@"Login failed !!!"];
    }
}