Hibernate followers and pains : Illegal attempt to associate a collection with two open sessions

i am working on a game project, where i need to rank players every 10 minutes, it is a simple! stock games. in order to update player ranks, i run a thread every 10 minutes , the thread fetch every player object(named player), calculates its score and then move on to next player and by the way i am storing the players score in its own object(player).

but i think it shouldn't be the cause, because MySql guarantees the data accuracy if the updates come inside a transaction block, so why should hibernate restrict this feature?

i don't know if this is a concurrency issue and has anything to do with the scenario that i told. and one thing else is i am not using c3p0! along with hibernate, i donno if this is related or not i just wanted to mention it.

thnx in forward,