.- Gemas utilizadas
  'simple_form'
  'select2-rails'

______________ View

<%= simple_form_for @model, validate: true do |f| %>

  <%= f.label :customers, 'Clientes' %>
  <%= f.collection_select :customer_ids, @producers, :id, :name_with_country, { :prompt => '- Select -' }, { :multiple => true, class: 'js-add-customers' } %>
  <%= f.submit "Guardar", class: "btn btn-block btn-primary" %>

<% end %>


______________ CoffeeScript

$('.js-add-customers').select2
    ajax:
      url: '/controller/customers.json'
      dataType: 'json'
      delay: 250
      data: (params) ->
        {
          q: params.term
          page: params.page
        }
      processResults: (data, page) ->
        { results: $.map(data, (customer, i) ->
          {
            id: customer.id
            text: customer.name
          }
        ) }
      cache: true
    theme: 'bootstrap'
    minimumInputLength: 1
    maximumInputLength: 20
    'width': '100%'

______________ Controller

def customers
  @customers = Customer.where('unaccent(lower(name) )LIKE unaccent(?)', "%#{params[:q].downcase}%")

  respond_to do |format|
    format.json { render json: @customers.map { |customer| { id: customer.id, name: customer.name_with_country } } }
  end
end