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db.collection.aggregate([{"$group":{"_id":{"clocked_in_at":{$dateTrunc:{date:{"$toDate":"$clocked_in_at"},unit:"day"}},"visitor_id":"$visitor_id"},"max":{"$max":"$clocked_out_at"},"min":{"$min":"$clocked_in_at"},"id":{"$first":"$id"}}},{"$project":{_id:"$id","visitor_id":"$_id.visitor_id","clocked_out_at":"$max","clocked_in_at":"$min"}}])