Override Doctrine ODM mapping for FOSUserBundle

ClassMetadataListener.php

<?php

namespace Acme\UserBundle\EventListener;

use Doctrine\ODM\MongoDB\Event\LoadClassMetadataEventArgs;

/**
 * Ran when Mongo metadata is loaded.
 */
class ClassMetadataListener
{
    public function loadClassMetadata(LoadClassMetadataEventArgs $eventArgs)
    {
        $classMetadata = $eventArgs->getClassMetadata();

        // Override FOS to not have unique emails
        if ($classMetadata->reflClass->name == 'FOS\UserBundle\Model\User') {
            foreach ($classMetadata->indexes as $i => $index) {
                if (count($index['keys']) === 1 && isset($index['keys']['emailCanonical'])) {
                    $classMetadata->indexes[$i]['options']['unique'] = false;
                    break;
                }
            }
        }
    }
}

services.yml

services:
    acme.user.metadata_listener:
        class: Acme\UserBundle\EventListener\ClassMetadataListener
        tags:
            -  { name: doctrine_mongodb.odm.event_listener, event: loadClassMetadata }