Custom HAML filter using Redcarpet to generate HTML with table of contents from Markdown. Copy file to config/initializers/ to install in Rails.

HAML_MarkdownWithTOC.rb

module Haml::Filters

  module MarkdownWithTOC

    include Haml::Filters::Base

    def render(text)
      <<~HAML_MD
      <nav class="toc">
        #{Redcarpet::Markdown.new( Redcarpet::Render::HTML_TOC.new ).render(text)}
      </nav>
      #{Redcarpet::Markdown.new( Redcarpet::Render::HTML
        .new( with_toc_data: true ) )
        .render(text)}
      HAML_MD
    end
  end

end