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db.collection.aggregate([{"$set":{"size_range":{"$cond":{"if":{"$lte":["$size",10000]},"then":[0,10000],"else":{"$cond":{"if":{"$lte":["$size",100000]},"then":[10000,100000],"else":[100000,"+inf"]}}}}}},{"$group":{"_id":{user:"$user",size:"$size_range"},"count":{"$sum":1}}},{"$project":{"user":"$_id.user","size":"$_id.size","count":"$count","_id":0}}])