ruby on rails f.select options with custom attributes

I have a form select statement, like this:

= f.select :country_id, @countries.map{ |c| [c.name, c.id] }

Which results in this code:

...
<option value="1">Andorra</option>
<option value="2">Argentina</option>
...

But I want to add a custom HTML attribute to my options, like this:

...
<option value="1" currency_code="XXX">Andorra</option>
<option value="2" currency_code="YYY">Argentina</option>
...