This is how add and use presenters in Rails

_partial_layout.erb

<% projects.each do |project| %>
  <% cache [project, current_locale] do %>
    <% present project do |project_presenter| %>
      <li class="container project">
        ....
        <div class="activities-overview">
          <ol class="activities">
            <%= project_presenter.show_categories %>
          </ol>
        </div>

        <div class="bar">
          <div class="graph">
            <%= project_presenter.show_categories_bar %>
          </div>
        </div>
        ....
      </li>
    <% end %>
  <% end %>
<% end %>
<%= paginate projects %>

Example: Presenter in Rails.rb

class ProjectPresenter
  NUMBER_OF_ACTIVITIES = 4
  attr_reader :template
  def initialize(project, template)
    @project = project
    @template = template
  end

  def show_categories
    template.render partial: "category",
                    collection: categories_with_remainder,
                    locals: { project: @project }
  end

  def show_categories_bar
    template.render partial: "categories_bar",
                    as: :category,
                    collection: categories_with_remainder,
                    locals: { project: @project }
  end

  private

  def sorted_categories
    @project.sorted_categories
  end

  def categories_with_remainder
    categories = sorted_categories
    if sorted_categories.length > NUMBER_OF_ACTIVITIES + 1
      categories = sorted_categories.take(NUMBER_OF_ACTIVITIES)
      remaining_categories = sorted_categories.drop(NUMBER_OF_ACTIVITIES)
      categories << RemainingCategory.new(remaining_categories)
    end
    categories
  end
end

helper.rb

module ApplicationHelper
  ....

  def present(object, klass = nil)
    klass ||= "#{object.class}Presenter".constantize
    presenter = klass.new(object, self)
    yield presenter if block_given?
    presenter
  end

  .....
end