A re-usable Versionable Behavior for Doctrine 2

My previous post on behaviors in Doctrine 2 generated quite some discussion about the difference on behaviours that are re-usable across models and the trivial specific implementations I have shown.

In this post I will show a re-usable versionable (audit-log) behavior. For this we will need the following ingredients:

• An interface DoctrineExtensions\Versionable\Versionable
• A class DoctrineExtensions\Versionable\VersionManager
• An event listener DoctrineExtensions\Versionable\VersionListener
• A generic entity DoctrineExtensions\Versionable\ResourceVersion

The Event API is currently in the central focus of our efforts so the API shown here may change before the first Beta release.

The workflow is as follows, each Entity that is supposed to be versionable has to implement the interface Versionable which looks like this:

<?php

namespace DoctrineExtensions\Versionable;

interface Versionable
{
    /**
     * @return int
     */
    public function getCurrentVersion();

    /**
     * @return array
     */
    public function getVersionedData();

    /**
     * @return int
     */
    public function getResourceId();
}

Whenever an entity is persisted or updated the state that is persisted will also be logged in an audit table. The state is returned with an array of key value pairs in the getVersionedData() and the current version has to be the value of the @Version column of the entity.

To sum up, the requirements of an entity that can be a Versionable in this simple implementation:

• Single Integer Primary Key.
• Has to be versioned with an integer column.

How does such versioned data look like? The generic resource version entity looks like this. Its a Doctrine Entity, but in a domain model its an immutable value object. It should not be changed after creation.

<?php

namespace DoctrineExtensions\Versionable;

class ResourceVersion
{
    /** @Id @Column(type="integer") */
    private $id;

    /** @Column(type="string") */
    private $resourceName;

    /** @Column(type="integer") */
    private $resourceId;

    /** @Column(type="array") */
    private $versionedData;

    /**
     * @Column(type="integer") */
    private $version;

    /** @Column(type="datetime") */
    private $snapshotDate;

    public function __construct(Versionable $resource)
    {
        $this->resourceName = get_class($resource);
        $this->resourceId = $resource->getResourceId();
        $this->versionedData = $resource->getVersionedData();
        $this->version = $resource->getCurrentVersion();
        $this->snapshotDate = new \DateTime("now");
    }

    // getters
}

Now we need to solve the problem of generating the ResourceVersion whenever an Versionable entity is persisted or updated. This can be done by using the Doctrine EventManager API. We will implement the EventSubscriber interface and hook into the "onFlush" event.

<?php

namespace DoctrineExtensions\Versionable;

use Doctrine\Common\EventSubscriber,
    Doctrine\ORM\Events,
    Doctrine\ORM\Event\OnFlushEventArgs,
    Doctrine\ORM\EntityManager;

class VersionListener implements EventSubscriber
{
    public function getSubscribedEvents()
    {
        return array(Events::onFlush);
    }

    public function onFlush(OnFlushEventArgs $args)
    {
        $em = $args->getEntityManager();
        $uow = $em->getUnitOfWork();

        foreach ($uow->getScheduledEntityInsertions() AS $entity) {
            if ($entity instanceof Versionable) {
                $this->_makeSnapshot($entity);
            }
        }

        foreach ($uow->getScheduledEntityUpdates() AS $entity) {
            if ($entity instanceof Versionable) {
                $this->_makeSnapshot($entity);
            }
        }
    }

    private function _makeSnapshot($entity)
    {
        $resourceVersion = new ResourceVersion($entity);
        $class = $this->_em->getClassMetadata(get_class($resourceVersion));

        $this->_em->persist( $resourceVersion );
        $this->_em->getUnitOfWork()->computeChangeSet($class, $resourceVersion);
    }
}

How do we hook this VersionListener into the EntityManager? We will wrap the VersionManager around it that handles registration and offers some convenience methods to retrieve the versions of a resource.

<?php

namespace DoctrineExtensions\Versionable;

use Doctrine\ORM\EntityManager;

class VersionManager
{
    private $_em;

    public function __construct(EntityManager $em)
    {
        $this->_em = $em;
        $this->_em->getEventManager()->addEventSubscriber(
            new VersionListener()
        );
    }

    public function getVersions(Versionable $resource)
    {
        $query = $this->_em->createQuery(
            "SELECT v FROM DoctrineExtensions\Versionable\ResourceVersion v INDEX BY v.version ".
            "WHERE v.resourceName = ?1 AND v.resourceId = ?2 ORDER BY v.version DESC");
        $query->setParameter(1, get_class($resource));
        $query->setParameter(2, $resource->getResourceId());

        return $query->getResult();
    }
}

Now using this to retrieve all the versions of a given entity that is versionable you would go and:

<?php

// $em EntityManager, $blogPost my Blog Post

$versionManager = new VersionManager($em);
$versions = $versionManager->getVersions($blogPost);

echo "Old Title: ".$versions[$oldVersionNum]->getVersionedData('title');

// Create a new version
$blogPost->setTitle("My very new title");    
$em->flush();

This is a first example of how to use the powerful Doctrine 2 Event API. It is certainly not easy to use, as you need to understand the inner workings of the UnitOfWork and the different steps it is in during the flush process. However you can generate huge benefits in reusability.

The versionable behaviour could be extended by the following features:

• Create a new interface Revertable that extends Versionable and add a method revert(Revertable $resource, $toVersion) to the VersionManager that handles the retrieval, invoking of revert and such.
• Create a new interface Diffable with a method diff($aVersion, $bVersion) and new method diff(Diffable $resource, $aId, $bId) to the VersionManager that handles the delegation of a difference computation between two versions to the Diffable implementor.

Another approach would be not to save the complete state of an entity during the flush operation, but only the fields that changed. This is generally called an AuditLog. We could add an Auditable interface much in the same manner than the Versionable and retrieve the ChangeSets of each entity during flush using the following event listener:

<?php

class AuditListener implements EventSubscriber
{
    public function getSubscribedEvents()
    {
        return array(Events::onFlush);
    }

    public function onFlush(OnFlushEventArgs $args)
    {
        $em = $args->getEntityManager();
        $uow = $em->getUnitOfWork();

        $changeDate = new DateTime("now");
        $class = $em->getClassMetadata('DoctrineExtensions\Auditable\AuditEntry');

        foreach ($uow->getScheduledEntityUpdates() AS $entity) {
            if ($entity instanceof Auditable) {
                $changeSet = $uow->getEntityChangeSet($entity);

                foreach ($changeSet AS $field => $vals) {
                    list($oldValue, $newValue) = $vals;
                    $audit = new AuditEntry(
                        $entity->getResourceName(),
                        $entity->getId(),
                        $oldValue,
                        $newValue,
                        $changeDate
                    );

                    $em->persist($audit);
                    $em->getUnitOfWork()
                       ->computeChangeSet($class, $audit);
                }
            }
        }
    }
}

This approach can also be re-used or combined with several similiar behaviours, like Taggable, Blamable, Commentable.