In a single transaction how many time i can use the Flush method(), is there any constaint on this.
For ex.

BeginTransaction
Session.SaveOrUpdate(object);
Session.Flush();
Session.Delete(object);
Session.Flush();
CommitTransaction;

Nope, no constraint. Every time you call flush it will execute any pending SQL statements required to flush the memory state of your session to the database. If you change nothing then flush will flush exactly that - nothing.

Symon.