<!-- contacts.xml -->
<property
    name="categoryId"
    visibility="never"
    translation="sulu_category.categories"
>
    <field-name>id</field-name>
    <entity-name>Sulu\Bundle\CategoryBundle\Entity\Category</entity-name>

    <joins>
        <join>
            <entity-name>Sulu\Bundle\CategoryBundle\Entity\Category</entity-name>
            <field-name>%sulu.model.contact.class%.categories</field-name>
        </join>
    </joins>

    <filter type="selection">
        <params>
            <param name="displayProperty" value="name" />
            <param name="resourceKey" value="categories" />
            <!-- add the requestParameters and the rootKey here -->
            <param name="requestParameters" type="collection">
                <param name="rootKey" value="{the key of the category}" />
            </param>
        </params>
    </filter>
</property>