How can I Filter Popup on relationship?

I am working with three modules:

Account

Contacts

WContracts (custom)

On WContracts I have Account and Contact, both appear as a relate filed, both are the result of a 1-M relationship.

Accounts - Contacts is N-M

When an Account is selected and the user goes to choose the Contact, I want to restrict the Contacts available to only those on the Account. 

I tried working through the example on the Developer Guide:

http://support.sugarcrm.com/Documentation/Sugar_Developer/Sugar_Developer_Guide_7.5/UI_Model/Views/Filters/index.html#Ad… 

But I can't quite get it to work or understand the examples.

The hardcoded filter in the example is of no use in this scenario, I need the value to be dynamic.

And the dynamic filter example is for actual fields, but I don't have a Contact field on Account or an Account field on Contact, I have a relationship between Accounts and Contacts. If I used the Contacts popup as is the only think I have to go by is a non-db field with "ContactRole at Account" which contains the Account name and the custom ContactRole (a custom module where ContactRole has two relate fields: Contact and Account).

What I'm really trying to do is to have filtered collection by relationship... not by search field.

So maybe I should be looking at:

http://support.sugarcrm.com/Documentation/Sugar_Developer/Sugar_Developer_Guide_7.5/UI_Model/Views/Filters/index.html#Ad… 

But it's still unclear to me how to use the relationship Accounts<->Contacts for the drawer filter.

Could I override the on click event on the Contacts Relate field to open a list of pre-filtered Contacts to select from instead of the standard select2 with the search option?

Because in that case I could use an API call to generate the collection:

url = app.api.buildURL('Accounts/'+account_id+'/link/accounts_contacts/')

and pass the resulting collection 

contactsCollection = app.data.createBeanCollection( "Contacts", data.records);

to an app.drawer.open with layout selection-list and collection contactsCollection

 

 

thanks,
FrancescaS