a_query.rb

# grab a page
page = Page.find(1)
# grab the page's list of content_items and eager load each page's markups
content_items = page.content_items.includes(:markups)
# first content_item
content_item = content_items.first

# returns a collection of markups - doesn't output a SQL query
content_item.markups

# returns the list of markup types - DOES output a SQL query for the markups
content_item.html

content_item.rb

class ContentItem < ActiveRecord::Base
  has_many :markups

  def html
    markups.map do |markup|
      markup.type
    end
  end
end

markup.rb

class Markup < ActiveRecord::Base
  # has start_index, end_index and type
end

page.rb

class Page < ActiveRecord::Base
  has_many :content_items
end