How to make an inner join for a drop down?

[Patrick]

How can I do a join for a drop-down?  I need to drop down all
categories from a specific catalog, so I need to do a join
ProdCatalogCategory and ProductCategory.   Can this be done with an
entity-constraint?  What is the appropriate way to do this for a
drop-down?

<field name="contentTypeId" position="1">
      <drop-down allow-empty="true">
           <entity-options description="${categoryName}"
entity-name="ProductCategory" key-field-name="productCategoryId">
                <!--    need to filter on a particular productCatalogId which I
need to get from  ProdCatalogCategory-->
            </entity-options>
       </drop-down>
</field>

Thanks.  Patrick