application_helper.rb

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

base_presenter.rb

class BasePresenter
  def initialize(object, template)
    @object = object
    @template = template
  end

private

  def self.inherited(klass)
    name = klass.name.match(/(\w+)Presenter/).captures.first.underscore
    define_method(name) { @object }
  end

  def method_missing(*args, &block)
    @template.send(*args, &block)
  end
end

show.html.haml

- if @stock.present?
  - present @stock do |stock|
    #stock
      %h1= t('stock.title', name: stock.name)
      %p= t('stock.value', value: stock.value)
      = stock.history_chart

stock_presenter.rb

class StockPresenter < BasePresenter
  delegate :name, :to => :stock

  def value
    stock.value ? number_to_currency(stock.value) : t('unknown')
  end

  def history_chart(size = :big)
    render 'stocks/history_chart', stock: stock, size: size
  end
end