Rails - content_tagでビューヘルパーを自作する。 ref: http://qiita.com/mnishiguchi/items/d7a225e89806666e48fc

application_helper.rb

module ApplicationHelper

  # kv_names:   配列 [key名, value名]
  # collection: hash { key1: value1, key2: value2, ... }
  def kv_table_for(kv_names, collection={})
    thead = content_tag(:thead) do
      content_tag(:tr) do
        tags = kv_names.map { |column| content_tag(:th, column, class: "text-center") }
        safe_join tags
      end
    end

    tbody = content_tag(:tbody) do
      tags = collection.map do |k, v|
        content_tag(:tr, class: "text-center") do
          concat content_tag(:td, k)
          concat content_tag(:td, v)
        end
      end
      safe_join tags
    end

    content_tag(:div, class: "table-responsive") do
      content_tag(:table, class: "table table-bordered table-hover table-striped") do
        thead + tbody
      end
    end
  end
end

show.html.haml

= kv_table_for %w(category volume), @items.group(:category).sum(:volume)