gistfile1.txt

 var map = function() { emit(this.userId, 1); } // emit samples by userId with value of 1
 var reduce = function(key, values) 
                      	{ return Array.sum(values); } // sum the total samples per user
 db.activities.mapreduce(map, reduce, 
  	  	{ query: { time: { $gt: NOW-1H } }, // time bound the query to the last hour
		      out: { replace: "lastHourUniques", sharded: true }) // persist the output to a collection