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db.collection.aggregate([{$match:{uID:{$in:["20200308","20200306"]}}},{$sort:{uID:1,"timeStamp":-1}},{$group:{_id:"$uID",ts:{$push:"$$ROOT"}}},{$project:{ts:{$slice:["$ts",30]}}},{$unwind:"$ts"},{$group:{_id:"$_id",sumcapLast30:{$sum:"$ts.capacityLeft"}}}])