Lock a relationship when creating new records

I'm building an app for clients which could be at same facility, in different cities. But each city could have more than one facility...

I currently have Clients related to facility as one to many, and facility to city as many to one... as the city for each facility will always be the same, how to I lock that down?

Say a new client is coming into a facility... I create a new facility, but I want the city to be prepopulated based on the selection of the facility.