Using group_by

announcement.rb

named_scope :for_summary,
 :joins => [:section, :emisor, :announcement],
 :order => "sections.weight, emisors.emisor_order, announcements.number"

bulletin.rb

def generate_summary
 self.announcements.for_summary
end

bulletins_controller.rb

def show
 @summary = @bulletin.generate_summary
end

show.html.erb

<% @summary.group_by(&:section).each do |section, announcements| %>
  <h2><%= section.name %></h2>
  <% announcements.group_by(&:emisor).each do |emisor, announcements| %>
    <h3><%= emisor.name %></h3>
    <% announcements.each do |announcement| %>
      <p><%= announcement.content %></p>
    <% end -%>
  <% end -%>
<% end -%>