lock_db_tables(); # lock tables and/or start a transaction 
                  # in a database independent way

# some preparation that requires a database lock

my $firsttime = 1;
foreach my $id (@list_o_object_ids) {
   lock_db_tables() unless $firsttime;
   $firsttime = 0;

   my $obj = new Thingy($id); # gets stuff from db

   # make changes, save back to db.

   unlock_tables();
}
[download]

I guess it's the unlock_tables() that you want to run on all except the last iteration, right? you can move it to the beginning of the loop and skip it in the first iteration:

my $firsttime = 1;
foreach my $id (@list_o_object_ids) {
   unless ($firsttime) {
      unlock_tables();
      lock_db_tables();
   } else {
      $firsttime = 0;
   }

   my $obj = new Thingy($id); # gets stuff from db

   # make changes, save back to db.
}
[download]

Jenda


We'd like to help you learn to help yourself Look around you, all you see are sympathetic eyes Stroll around the grounds until you feel at home    -- P. Simon in Mrs. Robinson

No, that's not right. I want to lock the tables on all but the first iteration. Or to view this problem another way, I want to do a heck of a lot (about 1100 lines) of work the first time through the loop that is not repeated subsequent times. Right now, the first-loop-specific stuff is not in the loop at all, and due to it's size I'd prefer to keep it outside the loop. This is not a problem except that the pre-loop work and the first loop iteration must be within a single database transaction, while each subsequent loop is also it's own transaction.

As I've already shown, (and you have shown a slightly different way) this problem can be solved with a semaphore variable and a check of the sempahore each trip through the loop. I just don't like that solution because it seems messy and wonder if there is a cleaner way. I was hoping to find some syntacical thingy that behaves kind of like a join, but with blocks of code rather than array elements. Here's some pseudo code to demonstrate what I had in mind.

lock_db_tables(); # lock tables and/or start a transaction 
                  # in a database independent way

# 1100 lines of preparation that requires a database lock
# must be in same transaction as first iteration of loop
# below

foreach my $id (@list_o_object_ids) {

   my $obj = new Thingy($id); # gets stuff from db

   # make changes, save back to db. (about 900 lines in my case)

   unlock_tables();
} join {
   lock_db_tables();
}
[download]

The lock_db_tables() within the join block would be executed between each loop iteration, but not before the first iteration, nor after the last iteration.

--DrWhy

"If God had meant for us to think for ourselves he would have given us brains. Oh, wait..."