mongodb "group by" query using aggregation for counting documents per category, on a objectid-timestamp-based subset

mostPostedTracks.js

db = db.getSiblingDB("whyd_music") // explicitely select collection
var jan2014 = ObjectId("52c35a800000000000000000"); // created using http://steveridout.github.io/mongo-object-time/
db.post.aggregate([
	{"$sort": {"_id": -1}}, // order: antichronological
	{"$match": {"_id": {"$gt": jan2014}}}, // only documents that were created after the 1syt january 2014
	{"$group": {"_id": "$eId", // group by value of the eId attribute -> _id attribute in the resulting output collection
	            "name": {"$first": "$name"}, // -> include the name of each grouped category
	            "count": {"$sum": 1}}}, // -> count attribute will contain the number of documents for each value of _eid
	{"$match": {"count": {"$gt": 50}}}, // limit output to results with count > 50
	{"$sort": {"count": -1}} // output order: highest count first
]).forEach(function(f){print(tojson(f, '', true));}); // avoids mongoshell's paging