Automatizando a seleção de Municípios/UF

application-lib.js

/**
 *  Campos de state com data-cities="nome_do_select_de_cidades"
 *  funcionarão automaticamente
 */
$('[data-cities]').change(function() {
    var $select = $('select[name="' + $(this).attr('data-cities') + '"]');
    var $value = $('input[type=hidden][name="' + $(this).attr('data-cities') + '"]');

    if (!$select.length) {
        return;
    }

    // Precisa selecionar um valor
    if(!this.value) {
        $select.html('').append('<option value="">Selecione a UF</option>');
        return;
    }

    $select.attr('disabled', 'true');
    $.getJSON('/cities/all_by_state_id/' + $(this).val(), function(data) {

        // Popula os dados no select de cidades
        $select = $select.html('').append('<option value="">Selecione</option>');
        $.each(data.data.cities, function(i, val){
            $select.append('<option value="' + val.id + '">' + val.name + '</option>');
        });
        $select.removeAttr('disabled');

        // Verifica se já tem uma opção pré-determinada
        var value = $select.attr('data-value') || $value.val();
        if (value) {
            $select.find('option[value=' + value + ']')
                .attr('selected', true)
                .removeAttr('data-value');
        }
    });
}).each(function() {
    var $cidade = $('select[name="' + $(this).attr('data-cities') + '"]').html('').append('<option value="">Selecione a UF</option>');
    var value = $cidade.attr('data-value');
    if (this.value || value) {
        $(this).change();
    }
});

application_helper.rb

def get_form_builder_field_name(object_name, field_name)
  if object_name.is_a?(ActionView::Helpers::FormBuilder)
    object_name = object_name.object_name
  end

  "#{object_name}[#{field_name.to_s.sub(/\?$/, '')}]"
end

CitiesController.rb

class CitiesController < ApplicationController
  def all_by_state_id
    uf = params[:id]
    cities = City.joins(:state).where('states.id' => uf).select('cities.id, cities.name')

    render :json => { data: { cities: cities } }
  end
end

layout.html.erb

<div class="control-group">
    <%= f.label :state_id, :class => 'control-label' %>
    <div class="controls">
        <%= f.collection_select :state_id, State.all, :id, :name, { :prompt => true }, :data => { :cities => get_form_builder_field_name(f, :city_id) } %>
    </div>
</div>

<div class="control-group">
    <%= f.label :city_id, :class => 'control-label' %>
    <div class="controls">
        <%= f.hidden_field :city_id %>
        <%= f.select :city_id, [] %>
    </div>
</div>