doctrine2/docs/en/reference/events.rst

Events
======

Doctrine 2 features a lightweight event system that is part of the
Common package. Doctrine uses it to dispatch system events, mainly
:ref:`lifecycle events <reference-events-lifecycle-events>`.
You can also use it for your own custom events.

The Event System
----------------

The event system is controlled by the ``EventManager``. It is the
central point of Doctrine's event listener system. Listeners are
registered on the manager and events are dispatched through the
manager.

.. code-block:: php

    <?php
    $evm = new EventManager();

Now we can add some event listeners to the ``$evm``. Let's create a
``EventTest`` class to play around with.

.. code-block:: php

    <?php
    class EventTest
    {
        const preFoo = 'preFoo';
        const postFoo = 'postFoo';

        private $_evm;

        public $preFooInvoked = false;
        public $postFooInvoked = false;

        public function __construct($evm)
        {
            $evm->addEventListener(array(self::preFoo, self::postFoo), $this);
        }

        public function preFoo(EventArgs $e)
        {
            $this->preFooInvoked = true;
        }

        public function postFoo(EventArgs $e)
        {
            $this->postFooInvoked = true;
        }
    }

    // Create a new instance
    $test = new EventTest($evm);

Events can be dispatched by using the ``dispatchEvent()`` method.

.. code-block:: php

    <?php
    $evm->dispatchEvent(EventTest::preFoo);
    $evm->dispatchEvent(EventTest::postFoo);

You can easily remove a listener with the ``removeEventListener()``
method.

.. code-block:: php

    <?php
    $evm->removeEventListener(array(self::preFoo, self::postFoo), $this);

The Doctrine 2 event system also has a simple concept of event
subscribers. We can define a simple ``TestEventSubscriber`` class
which implements the ``\Doctrine\Common\EventSubscriber`` interface
and implements a ``getSubscribedEvents()`` method which returns an
array of events it should be subscribed to.

.. code-block:: php

    <?php
    class TestEventSubscriber implements \Doctrine\Common\EventSubscriber
    {
        public $preFooInvoked = false;

        public function preFoo()
        {
            $this->preFooInvoked = true;
        }

        public function getSubscribedEvents()
        {
            return array(TestEvent::preFoo);
        }
    }

    $eventSubscriber = new TestEventSubscriber();
    $evm->addEventSubscriber($eventSubscriber);

.. note::

    If you are familiar with the Symfony2 event manager, note that
    the array returned by getSubscribedEvents is different. For
    doctrine, the array values must be the event names, and the
    names are used as method names that will be called when the
    event occurs.

Now when you dispatch an event, any event subscribers will be
notified for that event.

.. code-block:: php

    <?php
    $evm->dispatchEvent(TestEvent::preFoo);

Now you can test the ``$eventSubscriber`` instance to see if the
``preFoo()`` method was invoked.

.. code-block:: php

    <?php
    if ($eventSubscriber->preFooInvoked) {
        echo 'pre foo invoked!';
    }

Naming convention
~~~~~~~~~~~~~~~~~

Events being used with the Doctrine 2 EventManager are best named
with camelcase and the value of the corresponding constant should
be the name of the constant itself, even with spelling. This has
several reasons:


-  It is easy to read.
-  Simplicity.
-  Each method within an EventSubscriber is named after the
   corresponding constant. If constant name and constant value differ,
   you MUST use the new value and thus, your code might be subject to
   codechanges when the value changes. This contradicts the intention
   of a constant.

An example for a correct notation can be found in the example
``EventTest`` above.

.. _reference-events-lifecycle-events:

Lifecycle Events
----------------

The EntityManager and UnitOfWork trigger a bunch of events during
the life-time of their registered entities.


-  preRemove - The preRemove event occurs for a given entity before
   the respective EntityManager remove operation for that entity is
   executed.  It is not called for a DQL DELETE statement.
-  postRemove - The postRemove event occurs for an entity after the
   entity has been deleted. It will be invoked after the database
   delete operations. It is not called for a DQL DELETE statement.
-  prePersist - The prePersist event occurs for a given entity
   before the respective EntityManager persist operation for that
   entity is executed. It should be noted that this event is only triggered on
   *initial* persist of an entity
-  postPersist - The postPersist event occurs for an entity after
   the entity has been made persistent. It will be invoked after the
   database insert operations. Generated primary key values are
   available in the postPersist event.
-  preUpdate - The preUpdate event occurs before the database
   update operations to entity data. It is not called for a DQL UPDATE statement.
-  postUpdate - The postUpdate event occurs after the database
   update operations to entity data. It is not called for a DQL UPDATE statement.
-  postLoad - The postLoad event occurs for an entity after the
   entity has been loaded into the current EntityManager from the
   database or after the refresh operation has been applied to it.
-  loadClassMetadata - The loadClassMetadata event occurs after the
   mapping metadata for a class has been loaded from a mapping source
   (annotations/xml/yaml).
-  preFlush - The preFlush event occurs at the very beginning of a flush
   operation. This event is not a lifecycle callback.
-  onFlush - The onFlush event occurs after the change-sets of all
   managed entities are computed. This event is not a lifecycle
   callback.
-  postFlush - The postFlush event occurs at the end of a flush operation. This
   event is not a lifecycle callback.
-  onClear - The onClear event occurs when the EntityManager#clear() operation is
   invoked, after all references to entities have been removed from the unit of
   work.

.. warning::

    Note that the postLoad event occurs for an entity
    before any associations have been initialized. Therefore it is not
    safe to access associations in a postLoad callback or event
    handler.


You can access the Event constants from the ``Events`` class in the
ORM package.

.. code-block:: php

    <?php
    use Doctrine\ORM\Events;
    echo Events::preUpdate;

These can be hooked into by two different types of event
listeners:


-  Lifecycle Callbacks are methods on the entity classes that are
   called when the event is triggered. They receives some kind of ``EventArgs``.
-  Lifecycle Event Listeners and Subscribers are classes with specific callback
   methods that receives some kind of ``EventArgs`` instance which
   give access to the entity, EntityManager or other relevant data.

.. note::

    All Lifecycle events that happen during the ``flush()`` of
    an EntityManager have very specific constraints on the allowed
    operations that can be executed. Please read the
    :ref:`reference-events-implementing-listeners` section very carefully
    to understand which operations are allowed in which lifecycle event.


Lifecycle Callbacks
-------------------

A lifecycle event is a regular event with the additional feature of
providing a mechanism to register direct callbacks inside the
corresponding entity classes that are executed when the lifecycle
event occurs.

.. code-block:: php

    <?php

    /** @Entity @HasLifecycleCallbacks */
    class User
    {
        // ...

        /**
         * @Column(type="string", length=255)
         */
        public $value;

        /** @Column(name="created_at", type="string", length=255) */
        private $createdAt;

        /** @PrePersist */
        public function doStuffOnPrePersist()
        {
            $this->createdAt = date('Y-m-d H:i:s');
        }

        /** @PrePersist */
        public function doOtherStuffOnPrePersist()
        {
            $this->value = 'changed from prePersist callback!';
        }

        /** @PostPersist */
        public function doStuffOnPostPersist()
        {
            $this->value = 'changed from postPersist callback!';
        }

        /** @PostLoad */
        public function doStuffOnPostLoad()
        {
            $this->value = 'changed from postLoad callback!';
        }

        /** @PreUpdate */
        public function doStuffOnPreUpdate()
        {
            $this->value = 'changed from preUpdate callback!';
        }
    }

Note that when using annotations you have to apply the
@HasLifecycleCallbacks marker annotation on the entity class.

If you want to register lifecycle callbacks from YAML or XML you
can do it with the following.

.. code-block:: yaml

    User:
      type: entity
      fields:
    # ...
        name:
          type: string(50)
      lifecycleCallbacks:
        prePersist: [ doStuffOnPrePersist, doOtherStuffOnPrePersistToo ]
        postPersist: [ doStuffOnPostPersist ]

XML would look something like this:

.. code-block:: xml

    <?xml version="1.0" encoding="UTF-8"?>

    <doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                              /Users/robo/dev/php/Doctrine/doctrine-mapping.xsd">

        <entity name="User">

            <lifecycle-callbacks>
                <lifecycle-callback type="prePersist" method="doStuffOnPrePersist"/>
                <lifecycle-callback type="postPersist" method="doStuffOnPostPersist"/>
            </lifecycle-callbacks>

        </entity>

    </doctrine-mapping>

You just need to make sure a public ``doStuffOnPrePersist()`` and
``doStuffOnPostPersist()`` method is defined on your ``User``
model.

.. code-block:: php

    <?php
    // ...

    class User
    {
        // ...

        public function doStuffOnPrePersist()
        {
            // ...
        }

        public function doStuffOnPostPersist()
        {
            // ...
        }
    }

The ``key`` of the lifecycleCallbacks is the name of the method and
the value is the event type. The allowed event types are the ones
listed in the previous Lifecycle Events section.

Lifecycle Callbacks Event Argument
-----------------------------------

.. versionadded:: 2.4

Since 2.4 the triggered event is given to the lifecycle-callback.

With the additional argument you have access to the
``EntityManager`` and ``UnitOfWork`` APIs inside these callback methods.

.. code-block:: php

    <?php
    // ...

    class User
    {
        public function preUpdate(PreUpdateEventArgs $event)
        {
            if ($event->hasChangedField('username')) {
                // Do something when the username is changed.
            }
        }
    }

Listening and subscribing to Lifecycle Events
---------------------------------------------

Lifecycle event listeners are much more powerful than the simple
lifecycle callbacks that are defined on the entity classes. They
allow to implement re-usable behaviors between different entity
classes, yet require much more detailed knowledge about the inner
workings of the EntityManager and UnitOfWork. Please read the
*Implementing Event Listeners* section carefully if you are trying
to write your own listener.

For event subscribers, there are no surprises. They declare the
lifecycle events in their ``getSubscribedEvents`` method and provide
public methods that expect the relevant arguments.

A lifecycle event listener looks like the following:

.. code-block:: php

    <?php
    use Doctrine\Common\Persistence\Event\LifecycleEventArgs;

    class MyEventListener
    {
        public function preUpdate(LifecycleEventArgs $args)
        {
            $entity = $args->getObject();
            $entityManager = $args->getObjectManager();

            // perhaps you only want to act on some "Product" entity
            if ($entity instanceof Product) {
                // do something with the Product
            }
        }
    }

A lifecycle event subscriber may looks like this:

.. code-block:: php

    <?php
    use Doctrine\Common\EventSubscriber;
    use Doctrine\Common\Persistence\Event\LifecycleEventArgs;

    class MyEventSubscriber implements EventSubscriber
    {
        public function getSubscribedEvents()
        {
            return array(
                'postUpdate',
            );
        }

        public function postUpdate(LifecycleEventArgs $args)
        {
            $entity = $args->getObject();
            $entityManager = $args->getObjectManager();

            // perhaps you only want to act on some "Product" entity
            if ($entity instanceof Product) {
                // do something with the Product
            }
        }

.. note::

    Lifecycle events are triggered for all entities. It is the responsibility
    of the listeners and subscribers to check if the entity is of a type
    it wants to handle.

To register an event listener or subscriber, you have to hook it into the
EventManager that is passed to the EntityManager factory:

.. code-block:: php

    <?php
    $eventManager = new EventManager();
    $eventManager->addEventListener(array(Events::preUpdate), new MyEventListener());
    $eventManager->addEventSubscriber(new MyEventSubscriber());

    $entityManager = EntityManager::create($dbOpts, $config, $eventManager);

You can also retrieve the event manager instance after the
EntityManager was created:

.. code-block:: php

    <?php
    $entityManager->getEventManager()->addEventListener(array(Events::preUpdate), new MyEventListener());
    $entityManager->getEventManager()->addEventSubscriber(new MyEventSubscriber());

.. _reference-events-implementing-listeners:

Implementing Event Listeners
----------------------------

This section explains what is and what is not allowed during
specific lifecycle events of the UnitOfWork. Although you get
passed the EntityManager in all of these events, you have to follow
these restrictions very carefully since operations in the wrong
event may produce lots of different errors, such as inconsistent
data and lost updates/persists/removes.

For the described events that are also lifecycle callback events
the restrictions apply as well, with the additional restriction
that you do not have access to the EntityManager or UnitOfWork APIs
inside these events.

prePersist
~~~~~~~~~~

There are two ways for the ``prePersist`` event to be triggered.
One is obviously when you call ``EntityManager#persist()``. The
event is also called for all cascaded associations.

There is another way for ``prePersist`` to be called, inside the
``flush()`` method when changes to associations are computed and
this association is marked as cascade persist. Any new entity found
during this operation is also persisted and ``prePersist`` called
on it. This is called "persistence by reachability".

In both cases you get passed a ``LifecycleEventArgs`` instance
which has access to the entity and the entity manager.

The following restrictions apply to ``prePersist``:


-  If you are using a PrePersist Identity Generator such as
   sequences the ID value will *NOT* be available within any
   PrePersist events.
-  Doctrine will not recognize changes made to relations in a pre
   persist event called by "reachability" through a cascade persist
   unless you use the internal ``UnitOfWork`` API. We do not recommend
   such operations in the persistence by reachability context, so do
   this at your own risk and possibly supported by unit-tests.

preRemove
~~~~~~~~~

The ``preRemove`` event is called on every entity when its passed
to the ``EntityManager#remove()`` method. It is cascaded for all
associations that are marked as cascade delete.

There are no restrictions to what methods can be called inside the
``preRemove`` event, except when the remove method itself was
called during a flush operation.

preFlush
~~~~~~~~

``preFlush`` is called at ``EntityManager#flush()`` before
anything else. ``EntityManager#flush()`` can be called safely
inside its listeners.

.. code-block:: php

    <?php

    use Doctrine\ORM\Event\PreFlushEventArgs;

    class PreFlushExampleListener
    {
        public function preFlush(PreFlushEventArgs $args)
        {
            // ...
        }
    }

onFlush
~~~~~~~

OnFlush is a very powerful event. It is called inside
``EntityManager#flush()`` after the changes to all the managed
entities and their associations have been computed. This means, the
``onFlush`` event has access to the sets of:


-  Entities scheduled for insert
-  Entities scheduled for update
-  Entities scheduled for removal
-  Collections scheduled for update
-  Collections scheduled for removal

To make use of the onFlush event you have to be familiar with the
internal UnitOfWork API, which grants you access to the previously
mentioned sets. See this example:

.. code-block:: php

    <?php
    class FlushExampleListener
    {
        public function onFlush(OnFlushEventArgs $eventArgs)
        {
            $em = $eventArgs->getEntityManager();
            $uow = $em->getUnitOfWork();

            foreach ($uow->getScheduledEntityInsertions() AS $entity) {

            }

            foreach ($uow->getScheduledEntityUpdates() AS $entity) {

            }

            foreach ($uow->getScheduledEntityDeletions() AS $entity) {

            }

            foreach ($uow->getScheduledCollectionDeletions() AS $col) {

            }

            foreach ($uow->getScheduledCollectionUpdates() AS $col) {

            }
        }
    }

The following restrictions apply to the onFlush event:


-  If you create and persist a new entity in "onFlush", then
   calling ``EntityManager#persist()`` is not enough.
   You have to execute an additional call to
   ``$unitOfWork->computeChangeSet($classMetadata, $entity)``.
-  Changing primitive fields or associations requires you to
   explicitly trigger a re-computation of the changeset of the
   affected entity. This can be done by either calling
   ``$unitOfWork->recomputeSingleEntityChangeSet($classMetadata, $entity)``.

postFlush
~~~~~~~~~

``postFlush`` is called at the end of ``EntityManager#flush()``.
``EntityManager#flush()`` can **NOT** be called safely inside its listeners.

.. code-block:: php

    <?php

    use Doctrine\ORM\Event\PostFlushEventArgs;

    class PostFlushExampleListener
    {
        public function postFlush(PostFlushEventArgs $args)
        {
            // ...
        }
    }

preUpdate
~~~~~~~~~

PreUpdate is the most restrictive to use event, since it is called
right before an update statement is called for an entity inside the
``EntityManager#flush()`` method.

Changes to associations of the updated entity are never allowed in
this event, since Doctrine cannot guarantee to correctly handle
referential integrity at this point of the flush operation. This
event has a powerful feature however, it is executed with a
``PreUpdateEventArgs`` instance, which contains a reference to the
computed change-set of this entity.

This means you have access to all the fields that have changed for
this entity with their old and new value. The following methods are
available on the ``PreUpdateEventArgs``:


-  ``getEntity()`` to get access to the actual entity.
-  ``getEntityChangeSet()`` to get a copy of the changeset array.
   Changes to this returned array do not affect updating.
-  ``hasChangedField($fieldName)`` to check if the given field name
   of the current entity changed.
-  ``getOldValue($fieldName)`` and ``getNewValue($fieldName)`` to
   access the values of a field.
-  ``setNewValue($fieldName, $value)`` to change the value of a
   field to be updated.

A simple example for this event looks like:

.. code-block:: php

    <?php
    class NeverAliceOnlyBobListener
    {
        public function preUpdate(PreUpdateEventArgs $eventArgs)
        {
            if ($eventArgs->getEntity() instanceof User) {
                if ($eventArgs->hasChangedField('name') && $eventArgs->getNewValue('name') == 'Alice') {
                    $eventArgs->setNewValue('name', 'Bob');
                }
            }
        }
    }

You could also use this listener to implement validation of all the
fields that have changed. This is more efficient than using a
lifecycle callback when there are expensive validations to call:

.. code-block:: php

    <?php
    class ValidCreditCardListener
    {
        public function preUpdate(PreUpdateEventArgs $eventArgs)
        {
            if ($eventArgs->getEntity() instanceof Account) {
                if ($eventArgs->hasChangedField('creditCard')) {
                    $this->validateCreditCard($eventArgs->getNewValue('creditCard'));
                }
            }
        }

        private function validateCreditCard($no)
        {
            // throw an exception to interrupt flush event. Transaction will be rolled back.
        }
    }

Restrictions for this event:


-  Changes to associations of the passed entities are not
   recognized by the flush operation anymore.
-  Changes to fields of the passed entities are not recognized by
   the flush operation anymore, use the computed change-set passed to
   the event to modify primitive field values.
-  Any calls to ``EntityManager#persist()`` or
   ``EntityManager#remove()``, even in combination with the UnitOfWork
   API are strongly discouraged and don't work as expected outside the
   flush operation.

postUpdate, postRemove, postPersist
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The three post events are called inside ``EntityManager#flush()``.
Changes in here are not relevant to the persistence in the
database, but you can use these events to alter non-persistable items,
like non-mapped fields, logging or even associated classes that are
directly mapped by Doctrine.

postLoad
~~~~~~~~

This event is called after an entity is constructed by the
EntityManager.

Entity listeners
----------------

.. versionadded:: 2.4

An entity listeners is a lifecycle listener classes used for an entity.

- The entity listeners mapping may be applied to an entity class or mapped superclass.
- An entity listener is defined by mapping the entity class with the corresponding mapping.

.. configuration-block::

    .. code-block:: php

        <?php
        namespace MyProject\Entity;

        /** @Entity @EntityListeners({"UserListener"}) */
        class User
        {
            // ....
        }
    .. code-block:: xml

        <doctrine-mapping>
            <entity name="MyProject\Entity\User">
                <entity-listeners>
                    <entity-listener class="UserListener"/>
                </entity-listeners>
                <!-- .... -->
            </entity>
        </doctrine-mapping>
    .. code-block:: yaml

        MyProject\Entity\User:
          type: entity
          entityListeners:
            UserListener:
          # ....

.. _reference-entity-listeners:

Entity listeners class
~~~~~~~~~~~~~~~~~~~~~~

An ``Entity Listener`` could be any class, by default it should be a class with a no-arg constructor.

- Different from :ref:`reference-events-implementing-listeners` an ``Entity Listener`` is invoked just to the specified entity
- An entity listener method receives two arguments, the entity instance and the lifecycle event.
- A callback method could be defined by naming convention or specifying a method mapping.
- When the listener mapping is not given the parser will lookup for methods that match with the naming convention.
- When the listener mapping is given the parser won't lookup for any naming convention.

.. code-block:: php

    <?php
    class UserListener
    {
        public function preUpdate(User $user, PreUpdateEventArgs $event)
        {
            // Do something on pre update.
        }
    }

To define a specific event listener method
you should map the listener method using the event type mapping.

.. configuration-block::

    .. code-block:: php

        <?php
        class UserListener
        {
            /** @PrePersist */
            public function prePersistHandler(User $user, LifecycleEventArgs $event) { // ... }

            /** @PostPersist */
            public function postPersistHandler(User $user, LifecycleEventArgs $event) { // ... }

            /** @PreUpdate */
            public function preUpdateHandler(User $user, PreUpdateEventArgs $event) { // ... }

            /** @PostUpdate */
            public function postUpdateHandler(User $user, LifecycleEventArgs $event) { // ... }

            /** @PostRemove */
            public function postRemoveHandler(User $user, LifecycleEventArgs $event) { // ... }

            /** @PreRemove */
            public function preRemoveHandler(User $user, LifecycleEventArgs $event) { // ... }

            /** @PreFlush */
            public function preFlushHandler(User $user, PreFlushEventArgs $event) { // ... }

            /** @PostLoad */
            public function postLoadHandler(User $user, LifecycleEventArgs $event) { // ... }
        }
    .. code-block:: xml

        <doctrine-mapping>
            <entity name="MyProject\Entity\User">
                 <entity-listeners>
                    <entity-listener class="UserListener">
                        <lifecycle-callback type="preFlush"      method="preFlushHandler"/>
                        <lifecycle-callback type="postLoad"      method="postLoadHandler"/>

                        <lifecycle-callback type="postPersist"   method="postPersistHandler"/>
                        <lifecycle-callback type="prePersist"    method="prePersistHandler"/>

                        <lifecycle-callback type="postUpdate"    method="postUpdateHandler"/>
                        <lifecycle-callback type="preUpdate"     method="preUpdateHandler"/>

                        <lifecycle-callback type="postRemove"    method="postRemoveHandler"/>
                        <lifecycle-callback type="preRemove"     method="preRemoveHandler"/>
                    </entity-listener>
                </entity-listeners>
                <!-- .... -->
            </entity>
        </doctrine-mapping>
    .. code-block:: yaml

        MyProject\Entity\User:
          type: entity
          entityListeners:
            UserListener:
              preFlush: [preFlushHandler]
              postLoad: [postLoadHandler]

              postPersist: [postPersistHandler]
              prePersist: [prePersistHandler]

              postUpdate: [postUpdateHandler]
              preUpdate: [preUpdateHandler]

              postRemove: [postRemoveHandler]
              preRemove: [preRemoveHandler]
          # ....



Entity listeners resolver
~~~~~~~~~~~~~~~~~~~~~~~~~~
Doctrine invoke the listener resolver to get the listener instance.

- An resolver allows you register a specific ``Entity Listener`` instance.
- You can also implement your own resolver by extending ``Doctrine\ORM\Mapping\DefaultEntityListenerResolver`` or implementing ``Doctrine\ORM\Mapping\EntityListenerResolver``

Specifying an entity listener instance :

.. code-block:: php

    <?php
    // User.php

    /** @Entity @EntityListeners({"UserListener"}) */
    class User
    {
        // ....
    }

    // UserListener.php
    class UserListener
    {
        public function __construct(MyService $service)
        {
            $this->service = $service;
        }

        public function preUpdate(User $user, PreUpdateEventArgs $event)
        {
            $this->service->doSomething($user);
        }
    }

    // register a entity listener.
    $listener = $container->get('user_listener');
    $em->getConfiguration()->getEntityListenerResolver()->register($listener);

Implementing your own resolver :

.. code-block:: php

    <?php
    class MyEntityListenerResolver extends \Doctrine\ORM\Mapping\DefaultEntityListenerResolver
    {
        public function __construct($container)
        {
            $this->container = $container;
        }

        public function resolve($className)
        {
            // resolve the service id by the given class name;
            $id = 'user_listener';

            return $this->container->get($id);
        }
    }

    // configure the listener resolver.
    $em->getConfiguration()->setEntityListenerResolver($container->get('my_resolver'));

Load ClassMetadata Event
------------------------

When the mapping information for an entity is read, it is populated
in to a ``ClassMetadataInfo`` instance. You can hook in to this
process and manipulate the instance.

.. code-block:: php

    <?php
    $test = new EventTest();
    $metadataFactory = $em->getMetadataFactory();
    $evm = $em->getEventManager();
    $evm->addEventListener(Events::loadClassMetadata, $test);

    class EventTest
    {
        public function loadClassMetadata(\Doctrine\ORM\Event\LoadClassMetadataEventArgs $eventArgs)
        {
            $classMetadata = $eventArgs->getClassMetadata();
            $fieldMapping = array(
                'fieldName' => 'about',
                'type' => 'string',
                'length' => 255
            );
            $classMetadata->mapField($fieldMapping);
        }
    }