Group and order array by frequency

offer.rb

  # Private: order companions by frequency.
  #
  # orders  - Array of hashes.
  #
  # Returns Hashmap.
  def companions(orders)
    orders.reduce([]) {|acc, o| acc + o[:unique_product_ids]}.group_by(&:itself).transform_values!(&:size)
          .sort{ |a,b| b.second <=> a.second }.first(15)
  end