Select data binding - extra filter condition on another table

Hello everyone.

I have developed a Form which is substantially made of subforms. This because I 'd like to have entities which have the same structure but different meanings. Just to be more clear: I have two entities: Courts and Legal offices. I modeled them as the same (to be simple, a code and a description). But I created a form "Court" and a form "Legal Office", both of them simply including a subform "entity" that contains two text filed (code and description).

By the way, from the list of "Courts" I would like to add a person (let's say a judge). If I follow the same example of Accounts|Contacts in the demo CRM bundled with the installation, I need to put a Select box at the top of the "Person" Form, where I can choose which Court he/she will belong to.

The point is that: If use the Default Form Option Binder and select the form "entity" I  can access to "description" but get all the entities (both legal offices and courts, while I'd like to see only courts in this case). 

On the opposite, if use the Default Form Option Binder and select the form "Court" I  can access only to the internal id which is not understandable.

I tried to se the Default Form Option Binder and select the form "entity", adding  "e.id in (SELECT id from app_fd_courts)" in the "Extra Filter Condition", but the only result is  a Java exception:

 org.hibernate.hql.ast.QuerySyntaxException: app_fd_courts is not mapped 
SELECT e FROM app_fd_entities e  WHERE e.id in (SELECT id from app_fd_courts) ORDER BY cast(e.customProperties.description as string)

Any suggestion?