A Doctrine listener that allows you to inject your own generated discriminator map rather than have to define it in an entity's class doc block with the @DiscriminatorMap() annotation.

doctrine-create-discriminator-map-listener.php

<?php
/**
 * The DiscriminatorMap() annotation in an entity's class doc comment can be
 * empty, but if it is, Doctrine will create a default discriminator map
 * using the lower-case short name of each sub-class as the discriminator
 * "type" name. This may be just fine for your purposes, but if not,
 * this listener provides a way to programmatically inject your own
 * discriminator map.
 *
 * To prevent Doctrine from creating the default discriminator map, you will
 * need to put at least one "type"="class" pair in the annotation. You can
 * just use the parent class itself for this.
 *
 * I do the count() check between the two discriminator maps because I found
 * that, for some reason, the map would be assigned more than once. I'll have
 * to re-confirm that sometime, though.
 *
 * I'm using Doctrine within the Symfony 2 framework here, and I'm using the
 * JMS DI Extra Bundle to define the service.
 */
namespace My\Doctrine\ORM\Listener;

use JMS\DiExtraBundle\Annotation as DI;
use Doctrine\ORM\Event\LoadClassMetadataEventArgs;

/**
 * @DI\Service("my.create_discriminator_map.listener", public=false)
 * @DI\DoctrineListener(events={"loadClassMetadata"})
 */
class CreateDiscriminatorMapListener
{
    /**
     * @var array
     */
    protected $discriminatorMap;

    /**
     * @param string[] $discriminatorMap
     * @DI\InjectParams({"discriminatorMap"=@DI\Inject("%my.discriminator.map%")})
     */
    public function __construct(array $discriminatorMap)
    {
        $this->discriminatorMap = $discriminatorMap;
    }

    /**
     * @param LoadClassMetadataEventArgs $args
     */
    public function loadClassMetadata(LoadClassMetadataEventArgs $args)
    {
        $cMeta = $args->getClassMetadata();

        if (
            $cMeta->name == 'My\\Bundle\\FooBundle\\Entity\\ParentEntity' &&
            count($cMeta->discriminatorMap) < count($this->discriminatorMap)
        ) {
            $cMeta->setDiscriminatorMap($this->discriminatorMap);
        }
    }
}