query time spent by domain using tractor

top-domains.js

var map = function() {
  var domain = this.info.url.match(/^https?:\/\/([^\/]*)/)[1];
  emit(domain, { duration: this.duration });
};

var reduce = function(key, values) {
  var result = { duration: 0 };
  values.forEach(function(value) {
    result.duration += value.duration
  });
  return result;
};

db.items.mapReduce(map, reduce, {
  query: { 'info.url': /^https?:\/\/([^\/]*)/ },
  out: 'domain_durations' }
);

db.domain_durations.find().sort({ 'value.duration': -1 });