custom-attributes-to-select-options.rb

Rails CAN add custom attributes to select options, using the existing options_for_select helper. You almost had it right in the code in your question. Using html5 data-attributes:

<%= f.select :country_id, options_for_select(@countries.map{ |c| [c.name, c.id, {'data-currency_code'=>c.currency_code}] }) %>

Adding an initial selection:

<%= f.select :country_id, options_for_select(@countries.map{ |c| [c.name, c.id, {'data-currency_code'=>c.currency_code}] }, selected_key = f.object.country_id) %>

If you need grouped options, you can use the grouped_options_for_select helper, like this (if @continents is an array of continent objects, each having a countries method):

<%= f.select :country_id, grouped_options_for_select(@continents.map{ |group| [group.name, group.countries.map{ |c| [c.name, c.id, {'data-currency_code'=>c.currency_code}] } ] }, selected_key = f.object.country_id) %>

Credit should go to paul @ pogodan who posted about finding this not in the docs, but by reading the rails source. http://pogodan.com/blog/2011/02/24/custom-html-attributes-in-options-for-select