ActiveRecord::Relation#subgroup

ar_subgroup.rb

# TODO make a gem, a module or whatever
class ActiveRecord::Relation

  # allows nesting of results of several groupings
  # with groups,
  # Payment.group(:system).group(:total_currency, :service_name).sum(:total_cents)
  # => {["payture_in_pay", "RUB", "payture"]=>976067825,
  #    ["payture_in_pay", "RUB", "payture_test_ru"]=>637500,
  #    ...
  #
  # but with subgroups:
  #
  # Payment.group(:system).subgroup(:total_currency, :service_name) {|g| g.sum(:total_cents) }
  # => {"payture_in_pay"=>{
  #       ["RUB", "payture"]=>976067825,
  #       ["RUB", "payture_test_ru"]=>637500,
  #    ...
  #
  # TODO use extending of resulting relation and override calculate function
  # it will work without a block.
  def subgroup(*subgroups)
    relation = self
    old_key_size = relation.group_values.size
    relation = relation.group(*subgroups) unless subgroups.blank?

    result = yield relation

    raise ArgumentError, "result.inspect isn't a Hash. Call aggregate function in block!" unless result.is_a? Hash
    return result if result.empty?
    return result if old_key_size == 0

    key_sample = result.first.first
    new_key_size = key_sample.is_a?(Array) ? key_sample.size : 1
    result.group_by {|key, value|
      old_key_size == 1 ? key[0] : key[0, old_key_size]
    }.transform_values {|groups|
      groups.map {|subkey, value|
        [
          (new_key_size - old_key_size == 1) ? subkey[-1] : subkey[old_key_size..-1],
          value
        ]
      }.to_h
    }
  end
end

class ActiveRecord::Base
  delegate :subgroup, to: :all
end