Trying to use nested form with a drop down box [RoR3.0.9]

Hi

The nested form has a drop down box populated from a join table, that
is, on a foreign key, country_id, in this case.

Docs say that the foreign key will be used to set the “selected”
element of the drop down box.

Can’t make it work & don’t why. It always defaults to list element 1.

Any help would be appreciated.

~ Thanks