ruby on rails f.select options with custom attributes

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. https://web.archive.org/web/20130128223827/http://www.pogodan.com/blog/2011/02/24/custom-html-attributes-in-options-for-select

This is not possible directly with Rails, and you'll have to create your own helper to create the custom attributes. That said, there are probably two different ways to accomplish what you want:

(1) Using a custom attribute name in HTML5. In HTML5 you are allowed to have custom attribute names, but they have to be pre-pended with 'data-'. These custom attributes will not get submitted with your form, but they can be used to access your elements in Javascript. If you want to accomplish this, I would recommend creating a helper that generates options like this:

<option value="1" data-currecy-code="XXX">Andorra</option>

(2) Using values with custom splitting to submit additional data. If you actually want to submit the currency-code, I would recommend creating your select box like this:

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

This should generate HTML that looks like this:

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

Which you can then parse in your controller:

@id, @currency_code = params[:country_id].split(':')

You could do this as follows:

= f.select :country_id, @countries.map{ |c| [c.name, c.id, { 'data-currency-code' => c.currency_code} ] }