Select2SimpleForm is a custom input for SimpleForm which wraps and simplifies the Select2 functionality, without messing around with JS stuff.

Assuming you are using SimpleForm and a working Select2 plugin (a fast way of doing so is installing and configuring a Rails Select2 plugin, like this one). After having done this, then:

Add this to your Gemfile:

gem 'select2_simple_form', github: 'lndl/select2_simple_form', tag: '0.7.3'

And this into your app/assets/javascripts/application.js (making sure that the following line have to be placed after Select2 JS):

//= require select2_simple_form

And you're done my dear friend!

All you're gonna do is to use SimpleForm inputs as: :select2.

Render a Select2 simple collection (selecting a single item):

<%= f.input :countries, as: :select2, collection: Country.all %>

Render a Select2 multiple collection (selecting several items):

<%= f.input :countries, as: :select2, collection: Country.all, multiple: true %>

Render a Select2 performing an AJAX request that finds and shows the data (selecting a single item).

<%= f.input :countries, as: :select2, ajax: 'http://route/to/get/data/for/input' %>

Render a Select2 performing an AJAX request that finds and shows the data (selecting several items).

<%= f.input :countries, as: :select2, ajax: 'http://route/to/get/data/for/input', multiple: true %>

Internationalization (i18n) can be achieved yielding a hash whose keys are equals to js format functions. There are two ways:

Using a initializer:

Select2SimpleForm.configure do |config|
  # Spanish messages
  config.i18n = {
    formatNoMatches:       "No se encontraron resultados",
    formatInputTooShort:   "Por favor ingrese :n: caracter/es",
    formatInputTooLong:    "Por favor elimine :n: caracter/es",
    formatSelectionTooBig: "Sólo puede seleccionar :limit: elemento/s",
    formatLoadMore:        "Cargando más resultados...",
    formatSearching:       "Buscando..."
  }
end

<%= f.input :persona_id, as: :select2, ajax: search_path, i18n: { formatNoMatches: 'No se ha encontrado a la persona buscada' } %>

i18n messages can have parameters (check out the 'global' example), like :n:, :limit:, in order to render the proper quantities in JS.