Controller resources that are agnostic of content type to be rendered.

products.rb

module Products
  # What if we want to differ the content between iOS and Web?
  class IndexResource
    def initialize(product_presenters, content_type)
      @product_presenters = product_presenters
      @content_type = content_type
    end

    def render(rendering_engine = RenderingEngine)
      rendering_engine.render(partial_path, product_presenters: product_presenters)
    end

    private

    def partial_path
      "/path/to/templates/index.#{content_type}.erb"
    end

    attr_reader :content_type, :product_presenters
  end

  class ShowResource
    def initialize(product_presenter, content_type)
      @product_presenter = product_presenter
      @content_type = content_type
    end

    def render(rendering_engine = RenderingEngine)
      rendering_engine.render(partial_path, product_presenter: product_presenter)
    end

    private

    def partial_path
      "/path/to/templates/show.#{content_type}.erb"
    end

    attr_reader :content_type, :product_presenter
  end
end

products_controller.rb

class ProductsController < ApplicationController
  def index
    products = Product.all
    product_presenters = ProductPresenter.wrap_many(products)

    Products::IndexResource.new(product_presenters, content_type).render
  end

  def show
    product = Product.find(params[:id])
    product_presenter = ProductPresenter.new(product)

    Products::ShowResource.new(product_presenter, content_type).render
  end

  private

  def content_type
    # switch on the requested format
    if json?
      :json
    elsif web?
      :html
    end
  end
end