symfony2如何保证序列号不被使用

I have a document management system in symfony2. Once user establishes my document entity it will be assigned a serial number.

This is how I find the serial to assign 1) Check in DB for highest serial in use 2) Assign it and flush entity.

I use this serial as an identifier for multiple versions of this entity so it is not unique to the DB.

How can I guarantee that the number assigned is truly unique? I would lock tables in flat php but not sure how to or if it's best practice in symfony2.

/**
 * Establishes a analysi entity.
 *
 */
public function establishAction(Request $request, Analysis $analysi)
{

    ...

    $analysi->setSerial( $this->getNewSerial() );//set a new serial number
    //TODO: How to confirm this is really not in use since there is no transaction locking going on here?
    $em->flush($analysi);

    ...

}


private function getNewSerial()
{            
    $em = $this->getDoctrine()->getManager();

    //get highest serial nb from established analysises
    $results = $em->createQuery("SELECT MAX(a.serial) FROM HazardlogBundle:Analysis a WHERE a.currentVersion = true")->getResult();
    $temp = $results[0];
    $max_serial = $temp[1];

    $new_serial = $max_serial + 1;

    return $new_serial;
}

---------------- UPDATE

I must obviously clearify: I can have...

entity A with serial 123 and version 1
entity B with serial 123 and version 2
entity C with serial 124 and version 1
entity D with serial 125 and version 1

What I am afraid of is that users simultaneously creates entity C and D, and so my controller could fetch the currently highest serial number in use for creating entity C, and before entity C is flushed to DB, the thread serving the user creating entity D will read highest number 123 out of the DB before entity C with serial 124 is written to DB. Thus I will end up with Entity C & D both having serial 124.

Perhaps I could define a unique index consisting of serial and version number together to sidestep this issue?

I found a way to create a unique index by combining two columns like this:

/**
 * Analysis
 *
 * @ORM\Table(name="analysis",uniqueConstraints={@ORM\UniqueConstraint(name="unique_version_serial", columns={"Version", "serial"})})
 * @ORM\Entity(repositoryClass="HazardlogBundle\Repository\AnalysisRepository")
 */
class Analysis

...

i think you must put the uniqueness logic on your entity with annotation.

src/HazardlogBundle/Entity/Analysis.php

    /**
     * @var string
     *
     * @ORM\Column(name="serial", type="integer", unique=true)
     */
    private $serial;

If you are using MySQL - you can use AUTO_INCREMENT on separate table. You can find more in mysql docs: http://dev.mysql.com/doc/refman/5.6/en/information-functions.html#function_last-insert-id, see "simulate sequences" part.

If you are using PostgreSQL / Oracle / etc, you can use sequences: https://www.postgresql.org/docs/9.5/static/functions-sequence.html

Also, you can use uuid4 and generate it in php. Here is very low chance to get duplicate key.

All cases (except mysql one) can be coded as separate code or as doctrine's @GeneratedValue. http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/basic-mapping.html#identifier-generation-strategies
MySQL excluded because only one AUTO_INCREMENT per table, and, probably, you are already used it for primary key.