model2 form

<%= nested_form_for [@model1, @model2 :html => { :class => 'form-horizontal' } do |f| %>
...
  <div class="control-group">
    <%= f.label :model3_id, :class => 'control-label' %>
    <div class="controls">
      <%= f.select :model3_id, @model1.models3.order(:name).collect { |c| [c.name, c.id]} %><br />
    </div>
  </div>
...