Metadata Drivers

The heart of an object relational mapper is the mapping information that glues everything together. It instructs the EntityManager how it should behave when dealing with the different entities.

Core Metadata Drivers

Doctrine provides a few different ways for you to specify your metadata:

XML files (XmlDriver)
Attributes (AttributeDriver)
PHP Code in files or static functions (PhpDriver)

Something important to note about the above drivers is they are all an intermediate step to the same end result. The mapping information is populated to Doctrine\ORM\Mapping\ClassMetadata instances. So in the end, Doctrine only ever has to work with the API of the ClassMetadata class to get mapping information for an entity.

.. note::

The populated ``ClassMetadata`` instances are also cached
so in a production environment the parsing and populating only ever
happens once. You can configure the metadata cache implementation
using the ``setMetadataCacheImpl()`` method on the
``Doctrine\ORM\Configuration`` class:

.. code-block:: php

    <?php
    $em->getConfiguration()->setMetadataCacheImpl(new ApcuCache());

All the drivers are in the Doctrine\ORM\Mapping\Driver namespace:

.. code-block:: php

<?php
$driver = new \Doctrine\ORM\Mapping\Driver\XmlDriver('/path/to/mapping/files');
$em->getConfiguration()->setMetadataDriverImpl($driver);

Implementing Metadata Drivers

In addition to the included metadata drivers you can very easily implement your own. All you need to do is define a class which implements the MappingDriver interface:

.. code-block:: php

<?php

declare(strict_types=1);

namespace Doctrine\Persistence\Mapping\Driver;

use Doctrine\Persistence\Mapping\ClassMetadata;

/**
 * Contract for metadata drivers.
 */
interface MappingDriver
{
    /**
     * Loads the metadata for the specified class into the provided container.
     *
     * @param class-string<T> $className
     * @param ClassMetadata<T> $metadata
     *
     * @return void
     *
     * @template T of object
     */
    public function loadMetadataForClass(string $className, ClassMetadata $metadata);

    /**
     * Gets the names of all mapped classes known to this driver.
     *
     * @return list<class-string> The names of all mapped classes known to this driver.
     */
    public function getAllClassNames();

    /**
     * Returns whether the class with the specified name should have its metadata loaded.
     * This is only the case if it is either mapped as an Entity or a MappedSuperclass.
     *
     * @param class-string $className
     *
     * @return bool
     */
    public function isTransient(string $className);
}

If you want to write a metadata driver to parse information from some file format we've made your life a little easier by providing the FileDriver implementation for you to extend from:

.. code-block:: php

<?php

use Doctrine\Persistence\Mapping\ClassMetadata;
use Doctrine\Persistence\Mapping\Driver\FileDriver;

class MyMetadataDriver extends FileDriver
{
    /**
     * {@inheritDoc}
     */
    protected $_fileExtension = '.dcm.ext';

    /**
     * {@inheritDoc}
     */
    public function loadMetadataForClass($className, ClassMetadata $metadata)
    {
        $data = $this->_loadMappingFile($file);

        // populate ClassMetadata instance from $data
    }

    /**
     * {@inheritDoc}
     */
    protected function _loadMappingFile($file)
    {
        // parse contents of $file and return php data structure
    }
}

.. note::

When using the ``FileDriver`` it requires that you only have one
entity defined per file and the file named after the class described
inside where namespace separators are replaced by periods. So if you
have an entity named ``Entities\User`` and you wanted to write a
mapping file for your driver above you would need to name the file
``Entities.User.dcm.ext`` for it to be recognized.

Now you can use your MyMetadataDriver implementation by setting it with the setMetadataDriverImpl() method:

.. code-block:: php

<?php
$driver = new MyMetadataDriver('/path/to/mapping/files');
$em->getConfiguration()->setMetadataDriverImpl($driver);

ClassMetadata

The last piece you need to know and understand about metadata in Doctrine ORM is the API of the ClassMetadata classes. You need to be familiar with them in order to implement your own drivers but more importantly to retrieve mapping information for a certain entity when needed.

You have all the methods you need to manually specify the mapping information instead of using some mapping file to populate it from.

You can read more about the API of the ClassMetadata classes in the PHP Mapping chapter.

Getting ClassMetadata Instances

If you want to get the ClassMetadata instance for an entity in your project to programmatically use some mapping information to generate some HTML or something similar you can retrieve it through the ClassMetadataFactory:

.. code-block:: php

<?php
$cmf = $em->getMetadataFactory();
$class = $cmf->getMetadataFor('MyEntityName');

Now you can learn about the entity and use the data stored in the ClassMetadata instance to get all mapped fields for example and iterate over them:

.. code-block:: php

<?php
foreach ($class->fieldMappings as $fieldMapping) {
    echo $fieldMapping['fieldName'] . "\n";
}