Grouped Select in admin django

[Luis Salvay]

I have a model "Company" having many to many relationships with Country and City while country and city have one to many relationship between them. The problem is that when loading an "enterprise" have to select the countries atienede the company and the cities they serve, but the list of cities is very long and also the cities are all mixed without distinguishing what country you are. What I'd like is to group the cities by country. It is exactly what django-smart-selects, only this plugin does not work in many to many relationships. Someone could help me adapt this plugin to work with many to many relationship or comment me if they think of another alternative. Thank you very much!