app/views/marco_internos/_form.html.erb
<%= simple_form_for(@marco_interno) do |f| %>
<%= f.error_notification %>
<div class="form-inputs boxed_form">
<% f.association :partido %>
<div class="form-group">
<%= f.fields_for :documentos do |builder| %>
<%= render 'documento_fields', :f => builder %>
<% end %>
<%= link_to_add_association 'add document', f, :documentos %>
</div
</div>
<div class="form-actions">
<%= f.button :submit %>
</div>
<% end %>