{a:1, b:1, c:2}.invert # => {1=>:b, 2=>:c}:aさんが泣いちゃいます。Hash[*{a:1, b:1, c:2}.group_by{|k,v| v}.map{|v,kv| [v,kv.map{|k,v| k}]}.flatten(1)] # => {1=>[:a, :b], 2=>[:c]}よかったね！:aさん！

ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-linux] ruby 1.9.3p448 (2013-06-27 revision 41675) [x86_64-linux] gem 2.1.9 gem-1.9 1.8.23 にて。http://stackoverflow.com/questions/15583114/qtruby-emit-does-not-work これ。ruby2.0では動か…