count.js

  var agg = [
    {$group: {
      _id: "$episodeId",
      
      // FAIL!!!! :(
      total: {$count: "episodeId"}
    }}
  ];

  Tracking.aggregate(agg, function(err, logs){
    if (err) { return def.reject(err); }

    console.log(logs);
  });

group-by-count.js

  var agg = [
    {$group: {
      _id: "$episodeId",
      
      // SUCCESS!!! :D
      total: {$sum: 1}
    }}
  ];

  Tracking.aggregate(agg, function(err, logs){
    if (err) { return def.reject(err); }

    console.log(logs);
  });

group.js

  var agg = [
    {$group: {
      _id: "$episodeId"
    }}
  ];

  Tracking.aggregate(agg, function(err, logs){
    if (err) { return def.reject(err); }

    console.log(logs);
  });