fetching postgresql data cubes with active record

cube.rb

def cube(relation, keys)
  rs = relation.group("cube (#{keys.join(', ')})").pluck(*keys, 'count(*)', "grouping(#{keys.join(', ')})")
  hs = {}
  rs.group_by(&:pop).each do |grouping, rows|
    hs[select_by_bitmask(grouping, keys)] =
      rows.map do |*indexes, value|
        [select_by_bitmask(grouping, indexes), value]
      end.to_h
  end
  hs
end

private

def select_by_bitmask(bitmask, array);
  result = []
  array.reverse.each_with_index do |el, i|
    result.push el if bitmask & 2**i == 0
  end
  result
end