As noted, the order of the arguments in the array of an $in clause does not reflect the order of how the documents are retrieved. That of course will be the natural order or by the selected index order as shown. If you need to preserve this order, then you basically have two options. So … Read more
Using aggregate is the right approach, but you need to $unwind the list array before applying the $match so that you can filter individual elements and then use $group to put it back together: db.test.aggregate([ { $match: {_id: ObjectId(“512e28984815cbfcb21646a7”)}}, { $unwind: ‘$list’}, { $match: {‘list.a’: {$gt: 3}}}, { $group: {_id: ‘$_id’, list: {$push: ‘$list.a’}}} ]) … Read more
You could do this with MapReduce: mr = db.runCommand({ “mapreduce” : “my_collection”, “map” : function() { for (var key in this) { emit(key, null); } }, “reduce” : function(key, stuff) { return null; }, “out”: “my_collection” + “_keys” }) Then run distinct on the resulting collection so as to find all the keys: db[mr.result].distinct(“_id”) [“foo”, … Read more
There are a couple of ways to do this. The first is with Date Aggregation Operators, which allow you to dissect the “date” values in documents. Specifically for “grouping” as the primary intent: db.collection.aggregate([ { “$group”: { “_id”: { “year”: { “$year”: “$created_at” }, “dayOfYear”: { “$dayOfYear”: “$created_at” }, “hour”: { “$hour”: “$created_at” }, “interval”: … Read more
The best way to do this is in version 4.2+ which allows using of aggregation pipeline in the update document and the updateOne, updateMany or update collection methods. Note that the latter has been deprecated in most if not all languages drivers. MongoDB 4.2+ Version 4.2 also introduced the $set pipeline stage operator which is … Read more
MongoDB 2.2’s new $elemMatch projection operator provides another way to alter the returned document to contain only the first matched shapes element: db.test.find( {“shapes.color”: “red”}, {_id: 0, shapes: {$elemMatch: {color: “red”}}}); Returns: {“shapes” : [{“shape”: “circle”, “color”: “red”}]} In 2.2 you can also do this using the $ projection operator, where the $ in a … Read more