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