gistfile1.js

db.events.mapReduce(
  function(){
     emit({category:this.category, uid:this.uid}, null);
  },
  function(key, values){
    return null;
  },
  {query: {type: "pageview"}}
).map_reduce(
  function(){
     emit(this._id.category, 1);
  },
  function(key, values){
     return Array.sum(values);
  },
  {out:'unique_by_category'}
)