GalleryHasMedia.phpcr.xml

<?xml version="1.0" encoding="utf-8"?>
<doctrine-phpcr-mapping xmlns="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping"
                        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                        xsi:schemaLocation="http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping
                        http://doctrine-project.org/schemas/odm/doctrine-mongo-mapping.xsd">

    <document name="Sandbox\MediaBundle\Document\GalleryHasMedia" referenceable="true" >

        <reference-one fieldName="media" strategy="weak" targetDocument="Sandbox\MediaBundle\Document\Media" />

        <id name="id" type="id">
            <generator strategy="parent" />
        </id>

    </document>

</doctrine-phpcr-mapping>