How to obtain the selected value from collection_select

Rails
1

Hi,
I’m want to obtain the selected value from collection_select and to use
it
to display something from the same table or other tables.
There is my short code :

country name <%= collection_select(:country, :country_id, @countries, :id, :name) %>

by selecting a country id, my code should display the number of
population, the capital
and so one.

Do you have any idea?

Thanks