pivot.js

db.foo.drop();

db.foo.insert( { actor: "Richard Gere", movies: ['Pretty Woman', 'Runaway Bride', 'Chicago'] })
db.foo.insert( { actor: "Julia Roberts", movies: ['Pretty Woman', 'Runaway Bride', 'Erin Brockovich'] })
map = function() {
  for(var i in this.movies){
    key = { movie: this.movies[i] };
    value = { actors: [ this.actor ] };
    emit(key, value);
  }
}

reduce = function(key, values) {
  actor_list = { actors: [] };
  for(var i in values) {
    actor_list.actors = values[i].actors.concat(actor_list.actors);
  }
  return actor_list;
}

printjson(db.foo.mapReduce(map, reduce, "pivot"));

db.pivot.find().forEach(printjson);