Put a flag when a user visit a topic : trouble

Suppose I have a sort of forum with topics that users/admins can visit/manage.

When a user visit it, first I'll check if the topic exist; after I put a flag for say to the system if the user has been visited the topic.

This would be the basic code :

1        if((isset($_GET['trackid'])) && (ctype_digit($_GET['trackid']))) {
2            $query=mysql_query("SELECT date, user, zone, artist, event, data, res, complete, notes FROM topic WHERE trackid='".$_GET['trackid']."'",$mydb);    
3            if(mysql_num_rows($query)!=0) {
4                echo "Ok, The topic exist! Now I can say to the system开发者_如何学编程 that I have visited the topic"
5                
6                if (isset($_SESSION['nickname'])) $insert=mysql_query("INSERT IGNORE INTO recent_adds (user, trackid) VALUES ('".$_SESSION['nickname']."','".$_GET['trackid']."')",$mydb);                 
7            }
8        }

Now, suppose that User A arrive to the line 4, and meanwhile an admin (User B) remove this topic (trought a special function for the admins, but this doesn't mean so much) : it will insert a inconsistent data to the database. (in fact that topic won't be exist anymore).

How the major forum/system that do this kind of operation get rid about this concurrency problem?

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You can use FOREIGN KEYS to disallow inserting anything which have been deleted. Then you also get the benefit of doing a cascading delete.

Another solution is to run a batch job which cleans out inconsistent data. Along the lines of this solution, do not really delete the topic. Just mark it as deleted with a status flag and delete it later.