How can I design an efficient moderation system for comments?

Question

Here's the job I want to do:

My project is a website where there will be a lot of comments. Those comments will be moderated: a moderator will connect, see comments and accept or refuse them.

I need those things:

• the comments won't have any "moderator" associated. Which implies that when a moderator connects, the system "assigns" the 20 first comments to this moderator then sends them via an AJAX/JSON request
• of course, this implies that those comments musn't be assigned to other moderators

How can I make a design so that I'll be sure "getting the 20 first comments for a moderator" can be an atomic operation? And there's another thing to think of (I don't know if it may have an impact or not on my problem): if there are say 10 moderators connected, one of them has 20 comments to validate, and this guy leaves his PC for the whole day. I may implement kindof a "timeout" for this moderator, then, when he tries to validate, a message will tell him "sorry this comment has already been moderated by another moderator", and the AJAX script will get a "fresh new 20 comments).

My first idea was the simplest idea:

• a moderator connects
• a jQuery AJAX script asks the server for 20 comments
• the server (1) makes immediately a request of the 20 first free comments to the database
• the server (2) "freezes" the 20 free comments
• the server (3) makes up a JSON response and sends it

But the operations (2) and (3) are not atomic, what happens if, in-between, another operator asks for the 20 first free comments? They both will get the 20 first free comments. That's not a good solution. I think this is a classical problem of atomic operation on db. Any idea how to design this (or where to look)?

Thank you very much indeed!