Filtering a link type field

I am working on a Membership Management System and I have a Doctype called trainers which has a field Region/Area. When an applicant (another doctype which also has a field Region/Area) is filling the form, the applicant must select a trainer (which is a link type). I want the selection to be filtered based on the Region/Area the applicant selects, so that the applicant is only able to see trainers in the region chosen but not all the trainers.

Hope this will help you.