gistfile1.txt

 var map = function() { emit(userId, this.itemId); } // emit samples as userId: itemId
 var reduce = function(key, values) 
                      { return values; } // Return the array of itemIds the user has bought
 db.activities.mapReduce(map, reduce, 
  { query: { type: “ORDER”, time: { $gt: NOW-24H }}, // include only activities of type order int he last day
    out: { replace: "lastDayOrders", sharded: true }) // persist the output to a collection