symfony2 doctrine table lock - database

Question

I am not sure this is the preferred method, but I want to present my solution and see if you symfony2 wizards out there have enlightening comments on this.
I am registering financial transactions in a table, and each user has their own series of serial numbers (i.e. each user's transaction table will start with 1).
I understand that this must be handled by code, and then I run the risk of having duplicate entries for a user if let's say two people would be logged on to the same user account registering transactions, or the user triggers multiple transaction writes at the same time and Doctrine were to do the SELECTs in both operations before the firs write fires...
$em->getConnection()->exec('LOCK TABLES transaction WRITE;'); //lock for write access
$results = $em->createQuery("SELECT MAX(t.serial) FROM ekonomiKassabokBundle:Transaction t WHERE t.user = $userId")->getResult();
$temp = $results[0];
$max_serial = $temp[1];
$new_serial = $max_serial + 1;
$entity->setSerial($new_serial);
$em->persist($entity);
$em->flush();
$em->getConnection()->exec('UNLOCK TABLES;');
The above code gives me...
SQLSTATE[HY000]: General error: 1100 Table 't0_' was not locked with LOCK TABLES
Or is this perhaps even overkill, should I just skip the table lock?

Answer 1

I eventually managed to find the solution, well... a solution.
Actually from what I understand it is pretty stupid : when you lock tables MySQL expects ALL the tables you will use until the unlock to be locked and this must happen in one LOCK TABLES statement.
Now Doctrine will systematically use table aliases for whatever reason and MySQL apparently can't figure out that the aliases refer to locked tables... so you have to specifically lock all aliases that will be used yourself!
Try:
$em->getConnection()->exec('LOCK TABLES transaction as t0_ WRITE;');
And if you have another error after this (it will happen if you do several queries while the table is locked), just keep adding locks to the additional aliases, for instance:
$em->getConnection()->exec('LOCK TABLES transaction as t0_ WRITE, transaction as t0 WRITE, transaction as t1 WRITE;');
Fortunately it seems doctrine always uses the same table aliases, so once you have got it down it should keep working!