Rails - Nov 2006 - need advice about race condition

There''s a field in my User model that sometimes needs bulk updates:

   User.update_all("cluster = #{new_cluster}", "cluster = # 
{current_cluster}")

Now, suppose user with id 37 is affected by that particular  
update_all and updates his profile:

   1. user 37 is read
   2. update_all happens and changes 37''s cluster asynchronously
   3. user 37 is saved

now user 37 belongs to the wrong cluster. Unlikely but possible. Is  
there a robust way to write that?

-- fxn