Enumerable#group_by_many

enum_group_by_many.rb

Enumerable.class_eval do
  def group_by_many
    {}.tap do |groups|
      each do |value|
        keys = yield value
        keys.each do |key|
          group = groups[key] ||= []
          group << value
        end
      end
    end
  end
end

example.rb

#!/usr/bin/env ruby

load 'enum_group_by_many.rb'
require 'awesome_print'

Show = Struct.new :name, :cast

# partial cast list for demo purposes :)
shows = [
  Show.new(:dr_horrible, [:neil_patrick_harris, :nathan_fillion, :felicia_day]),
  Show.new(:firefly, [:nathan_fillion, :jewel_staite, :summer_glau]),
  Show.new(:dollhouse, [:eliza_dushku, :summer_glau, :felicia_day]),
]

shows.group_by_many(&:cast).each do |actor, shows|
  puts "#{actor}: #{shows.map(&:name).join ', '}"
end

# Output:
#
# neil_patrick_harris: dr_horrible
# nathan_fillion: dr_horrible, firefly
# felicia_day: dr_horrible, dollhouse
# jewel_staite: firefly
# summer_glau: firefly, dollhouse
# eliza_dushku: dollhouse