app/views/backend/admin_users/_form.html.erb
<%= simple_form_for [:backend, resource] do |f| %>
<%= render 'shared/error_messages', object: f.object %>
<%= f.input :email %>
<%= f.association :roles, input_html: {multiple: true, class: 'chzn-select',
"data-pre" => f.object.roles.to_json(methods: :name), only: [:id, :name]} %>
<%= f.submit 'Crear Usuario' %>
<% end %>