regrouping activerecord's group of group of groups

regroup.rb

module Regroup

  module_function

  # Makes a tree out of a flat hashes with array keys.
  # Useful for transformation of things like
  # relation.group(:a).group(:b).group(:c).count
  # to a hash of hash of hahes of any depth.
  # Safe for usage with plain hashes, too.
  # 
  #   > rs = Payment.group(:date).group(:currency).sum
  #   => {[Tue, 19 Jan 2016, "EUR"] => 100,
  #       [Tue, 19 Jan 2016, "USD"] => 20,
  #       [Wen, 20 Jan 2016, "EUR"] => 6,
  #      }
  #
  #   > regroup(rs)
  #   => {
  #        Tue, 19 Jan 2016 => {
  #          "EUR" => 100,
  #          "USD" => 20
  #        },
  #        Wen, 20 Jan 2016 => {
  #          "EUR" => 6
  #        }
  #      }

  def regroup(hash)
    tree = {}
    hash.each do |(*heads, tail), value|
      heads.inject(tree) { |subtree, key|
        subtree[key] ||= {}
      }[tail] = value
    end
    tree
  end

end